0
Origami Nanofabrication of Three-Dimensional
Electrochemical Energy Storage Devices
by
Hyun Jin In
B.S., University of California, Berkeley (2003)
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
ASSCHUB
S INS
OF TECHNOLOGY
at the
at theJ
UN 16 2005
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LIBRARIES
June 2005
0 Massachusetts Institute of Technology 2005. All rights reserved.
Signature of Author................
*-B:rtnient of Mechanical Engineering
May 20, 2005
Certified by
Assistant.......o
Assistant Profies
............................
George Barbastathis
r of Mechanical Engineering
Thesis Supervisor
Accepted by
Lallit Anand
Chairman, Department Committee on Graduate Students
BARKER
E
Origami Nanofabrication of Three-Dimensional
Electrochemical Energy Storage Devices
by
Hyun Jin In
Submitted to the Department of Mechanical Engineering
on May 9, 2005, in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
The Nanostructured OrigamiTM 3D Fabrication and Assembly Process was developed as a
novel method of creating three-dimensional (3D) nanostructured devices using twodimensional micro- and nanopatterning tools and techniques. The origami method of
fabrication is a two-part process in which two-dimensional (2D) membranes are first
patterned and then folded into the desired 3D configuration. This thesis presents an
origami fabrication method based on the use of SU-8 membranes and elastic gold hinges.
Magnetic actuation, stress-induced folding, vertical spacing, and lateral alignment of the
membranes are discussed.
This thesis also reports on the used of the Nanostructured OrigamiTM process to create
a functional electrochemical energy storage device. An electrochemical capacitor, or a
supercapacitor, is selected because its performance can be readily improved by the
addition of 3D geometry and nanoarchitecture. In addition to improved performance, the
origami fabrication method allows such devices to be integrated into preexisting MEMS
and IC processes, thus enabling the fabrication of complete micro- and nanosystems with
an integrated power supply. The supercapacitors were created by selectively depositing
carbon-based electrode materials on the SU-8 membrane and then folding the structure so
that oppositely-charged electrode regions face each other in a 3D arrangement. The
fabrication process, electrochemical testing procedure, and analysis of the results are
presented.
Thesis Supervisor: George Barbastathis
Title: Assistant Professor
3
4
Acknowledgments
First and foremost, I give all thanks and glory to my Lord and Savior Jesus Christ. It is
only by the grace of God that I am here and able to carry on each day.
I would like to express my deepest gratitude to many people here at MIT who have
made this work possible. First of all, I would like to thank my advisor, Professor George
Barbastathis, for ...well... pretty much everything. The past two years have been simply
amazing, and I greatly look forward to the next couple of years.
I would also like to acknowledge and thank Professor Henry I. Smith, Professor Yang
Shao-Horn, and Dr. Sundeep Kumar for their invaluable contributions to my research.
Of course, I couldn't have done anything without the generous help of the staff at
MIT's Microsystems Technology Laboratories. In particular, I would like to acknowledge Bob Bicchieri, Kurt Broderick, Vicky Diadiuk, Dave Terry, and Paul Tierney for
lending me their fabrication expertise.
Much thanks also to everyone in our lab. Kehan, Laura, Nader, Paul, Pepe, Satsoshi,
Se Baek, Tony, Wenyang, Will, and Zao... you guys are awesome! I would like to
especially thank the members of the origami crew: Will, Tony, and Paul. Let's all be
there together when origami rules the world!
I must also mention my three roommates, a.k.a. "The Dream Team." You have made
my time at MIT so much more enjoyable, and I will cherish all of our exciting adventures
for a lifetime.
Last, but definitely not least, my family has been a constant source of love and encouragement. I would like to thank my parents, my little brother, and my grandparents for
their incredible support. Words cannot begin to explain my gratitude and love for you all.
sdg
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6
Contents
23
1 Introduction
1.1
3D Nanomanufacturing ......................................
24
1.1.1
Nanofabrication .......................................
24
1.1.2
Three-dimensional fabrication .............................
25
1.2
Overview of Nanostructured OrigamiTM process ....................
1.3
Electrochemical energy storage and conversion devices ..............
1.3.1
Advantages of 3D and nanoarchitecture .....................
.30
31
.
32
1.3.2 Electrochemical capacitor ................................
33
1.3.2 Advantages of origami fabrication for supercapacitors ..........
35
1.4 Thesis objectives ............................................
36
1.5 O utline of thesis ..............................................
37
2 General Design Criteria for Nanostructured OrigamiTM Devices
2.1
Functional requirem ents .......................................
39
39
2.1.1
Rigid membrane and hinge ...............................
40
2.1.2
Actuation ............................................
44
2.1.3 Alignment ............................................
49
2.1.4 L atching ..............................................
51
2.1.5
Interconnection ........................................
52
2.2 M aterial selection ...........................................
52
2.2.1
M embrane ............................................
52
2.2.2
Hinge m aterial .........................................
54
2.2.3
Electrode material ......................................
55
2.3
H inge design ................................................
7
56
2.3.1 Failure analysis.......................
57
2.3.2 Lorentz force actuation.................
. . . .. . . . .. . . . . . .. .
59
2.3.3
. . . . . . . . .. . . . .. . . .
62
.. . .
Strain mismatch considerations. .........
2.4 Pyram id structures..........................
......
... ...
64
2.4.1 Spacing and alignment.................
.. .. . .. .....
... ...
64
2.4.2 Increased surface area..................
. . . . . .. .. . . . . . . .. .
3 Fabrication
3.1
67
Fabrication process ..................
3.2 Processing details ..........................
3.2.1
66
SU-8 processing ......................
....... . . . . . .. . .. . . . . . . . .
67
.... ..... .... .... .
72
.. .. . .....
72
.. ... .. .
3.2.2 Release step .........................
......
3.2.3
..... ....... ......
81
3.2.4 Carbon electrode ......................
.. .......... ......
81
3.2.5
... ..... ...... ....
83
W afer dicing .........................
Packaging ...........................
............
4 Fabrication Results and Testing
78
85
4.1 Released devices ...........................
. . . . . . .. . . . . . . . . . .
85
.. .... ..... .... ...
85
4.1.2 One-flap supercapacitor devices .
. . . .. .. . . .. . . . . . . .
89
4.1.3 Elastic spring-back ...........
.... ..... ...... ...
92
4.2 Pyramid structures .................
. . .. . .. . . . . . . . . . . .
94
.....
94
4.1.1
4.2.1
Two-flap supercapacitor devices
Increased surface area .........
4.2.2 Spacing and alignment ........
4.3 A ctuation ........................
4.3.1
Magnetic actuation ........
4.3.2
Stressed-induced actuation .....
..
4.4 Latching .........................
4.4.1
Mechanical latching ........
..
4.4.2 Photoresist latching ...........
... ...
. . . . . . . . .. . . .. . . . .
95
. ...... ...... .....
98
.. .... ..... .... ...
98
... ...............
104
. . .....
... .. ... ...
107
. . .....
.....
... ...
107
... .. .
10 8
. .. .. ... . .
4.5 Second-generation supercapacitor devices ..........
5 Electrochemical Testing
.. .....
110
113
8
5.1 Experimental process and setup .................................
113
5.2 Experimental results and discussion ..............................
115
6 Conclusions and Future Work
121
A Electrochemical Testing Methods
125
A.1
Experimental process and setup .................................
125
A.2 Experimental results and discussion ..............................
126
A.2 Experimental results and discussion ..............................
126
B Process Flow for First-Generation Supercapacitor Devices
129
C Mask Layout for First-Generation Supercapacitor Devices
133
D Process Flow for Second-Generation Supercapacitor Devices
137
E Mask Layout for Second-Generation Supercapacitor Devices
139
9
10
List of Figures
1-1
Cross-sectional SEM image of a 3D photonic crystal created using a
lithographic layer-by-layer approach. Seven functional layer can be seen
26
[10 ] . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1-2
3D photonic crystal structure created via highly precise stacking [11]. (a)
Schematic drawing of a four-layer structure. (b) SEM image of a two-layer
structure ..................................................
1-3
..
26
SEM image of a car fabricated using microstereolithography. The car is
27
approximately 2 mm in length and around 3 hours to complete [13] ......
1-4
SEM image of a 3D photonic crystal fabricated via holographic lithography. Four non-coplanar laser beams were used to create a 3D interference
28
pattern in a layer of SU-8 [15] ....................................
1-5
SEM images of complex 3D shapes created with two-photon absorption
polymerization. (a) A microbull with 150-nm minimum feature size.
Fabrication time is approximately 3 hours [18] (b) Cross section of 3D
photonic crystal made from SU-8 [19]............................
1-6
.
SEM images of 3D structures formed by colloidal assembly of microspheres [22]. (a) Face-centered cubic lattice synthetic opal template formed
by self-organization of 855-nm spheres. (b) Silicon photonic crystal
formed by conformal filling of the opal template .....................
11
.29
29
1-7
SEM images of 3D microstructures created with a single-step assembly
technique [24]. (a) Corner cube retroflector (CCR) after manual flipping of
one plate. (b) Pop-up box that is closed on all four sides ...............
1-8
.30
Conceptual drawings illustrating the Nanostructured OrigamiTM process.
(a) During the first stage of the process, planar fabrication methods are
used to pattern a 2D membrane. (b) Various actuation and alignment
mechanisms are used to automatically fold the 2D membrane into a 3D
configuration. (c) The final nanopattemed 3D devices .................
1-9
.31
Principle of a double-layer capacitor. Electrolytic solution spreads throughout the porous carbon structure, and charge is accumulated at the resulting
electrode/electrolyte interface [48] . . . . . . . . . . . . . . . . . . . . . . . . .
34
1-10
Ragone plot for different energy storage and conversion devices [48] .....
34
1-11
Drawing of a multi-layer supercapacitor with flexibility in voltage and
current outputs ...............................................
2-1
36
SEM image of a surface micromachined substrate hinge holding down a
horizontal polysilicon flap on the silicon substrate. The hinge is comprised
of two polysilicon layers [24] ....................................
2-2
41
Diagram of the strained hinge device before release. Etching all three
layers defines the shape of the device while etching only the top layer
creates flexible bending regions [52] ...............................
2-3
42
SEM images of PDMA devices [50]. (a) Permalloy defines a rigid layer on
top of the flexible gold layer. (b) Devices are bent at the god plastic
bending region ...............................................
2-4
SEM images of two polysilicon flaps connected with a photoresist hinge
[5 9 ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-5
42
43
SEM image of a micromirror raised via comb drive actuators and a complex mechanical driving mechanism [61] ...........................
12
45
2-6
Diagram of a flap being folded to 900 due to surface tension forces [59].
(a) A meltable material such as solder or photoresist is deposited at the
folding crease. (b) The deposited material is melted. (c) Surface energy
minimization of the melted material results in a deformation of the material
46
and a rotation of the flap .......................................
2-7
Diagram illustrating the idea of bimorph actuation. When the top (black)
layer shrinks in volume, the entire structured bends up to compensate for
47
the strain mismatch [69]........................................
2-8
SEM image of a self-assembled, out-of-plane inductor created with a
47
stress-engineered M oCr layer [70] ................................
Illustration of the PDMA process [50] ............................
. 48
2-10
Illustration of the Lorentz force actuation method [72] .................
.48
2-11
SEM images of (a) convex and (b) concave elements. The two features
2-9
mechanically couple to allow passive wafer alignment [74] .............
2-12
.50
Schematic cross section of micromechanical Velcro structures. When two
surfaces covered with these structures are pressed together, the tabs deform
and spring back to create an interlocked structure [80] .................
2-13
.51
Drawing showing the parameters 1, w, and t of the gold hinge that connects
two SU-8 segments. (Note: In the actual device, the SU-8 layer is above
the gold layer, not the other way aroundas shown in the illustration.) . . . .
2-14
Stress-strain curves for (a) an ideal elastic, perfectly plastic material and
(b) a ductile material that exhibits necking behavior ...................
2-15
57
.57
The series of drawings show what happens to the hinge during the release
process. As the silicon below the device is progressively etched away, the
lateral shrinkage of the SU-8 causes the hinges to be stretched ..........
2-16
58
Stress distribution diagrams for the bending of an elastic, perfectly plastic
material. (a) Fully elastic behavior. (b) After onset of plastic deformation.
(c) Fully plastic deform ation .....................................
13
60
2-17
Plot of bending angle vs. chromium thickness given the parameters in
Table 2.4 ..................................................
2-18
63
Fabrication of an inverted pyramidal pit using KOH etching. (a) The
masking layer is patterned to expose the silicon surface. (b) Etching in the
[100] direction takes place very rapidly while etching very slowly in the
[111] direction. (c) Once the { 111 } planes meet, the etching process is
effectively self-terminated as only the slow-etching { 111 } planes remain . .
2-19
64
Conceptual drawings illustrating how pyramid structures could be used to
improve spacing and alignment. (a) The top flap is folded over and brought
into contact with the bottom flap. (b) Corresponding square openings on
the top flap fit tightly over the pyramids on the bottom layer and insure
correct spacing and alignment between the two membranes .............
2-20
.65
As the top membranes is brought into contact with the bottom membrane,
the mechanical coupling between the square opening on the top layer and
the pyramid on the bottom layer forces the top layer into alignment and
prevents further downward movement ............................
3-1
.
65
Side profile illustration of the process flow for the origami fabrication of
nanostructured electrochemical capacitors. (a) KOH is used to etch pyramidal cavities into the silicon substrate. (b) Metal layer for the hinges
and various wiring is deposited via e-beam evaporation and patterned with
wet etching. (c) SU-8 layer is spun on and patterned to serve as the structural material. (d) XeF 2 gas is used to isotropically etch away the underlying silicon and release the device ................................
3-2
Top view of the process flow shown in Figure 3-1 ....................
3-3
Folding and painting of a supercapacitor following release. (a) The re-
. 69
70
leased device after XeF 2 etching. (b) First fold reveals the gold electrode
surface, which can then be painted with a carbon paint mixture. (c) Second
fold brings together the painted surfaces to form one active electrochemical
cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
71
3-4 Probe station setup used for manual assembly of the origami supercapacito rs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-5
SEM image of a gold hinge that has been stretched and broken during the
XeF 2 release process ..........................................
3-6
72
74
Dimensions of a two-flap device that can change as a result of SU-8
shrinkage. The two stars indicate last points of release for the SU-8 flaps.
Shrinkage will occur with respect to these two anchor points ............
3-7
74
SEM images of unreleased, 15 pm thick, one-flap device fabricated (a)
without and (b) with the hard bake step. The hard bake step relieves some
of the stress in the top surface effectively removing surface cracks and
reducing w arping ..............................................
3-8
Microscope images of a gold surface (a) before and (b) after approximately
30 minutes in the XeF 2 etch chamber ..............................
3-9
78
78
SEM images of a gold hinge after XeF 2 etching. (a) The hinge is stretched
beyond failure and also severely etched. (b) The hinge is almost completely etched aw ay ............................................
79
3-10
SEM image of an intact 2 pm thick hinge after XeF 2 release ............
80
3-11
SEM image of the carbon paint mixture (99wt% Super P and 1 wt% PVDF)
showing its porous structure and nano-sized particles ..................
3-12
82
Microscope image of the carbon film left on a gold surface after all the
solvent is evaporated away .....................................
82
3-13
Image of the completed supercapacitor package, ready for testing ........
83
4-1
Microscope image of the two-flap supercapacitor device with two separate
current loops for Lorentz force folding of the two segments ............
4-2
86
Microscope image of the two-flap supercapacitor device without current
loops for Lorentz force actuation..................................
15
86
4-3
Microscope images of the two-flap supercapacitor device upon complete
release viewed from the (a) top and from the (b) side. The carbon paint has
not yet been applied ...........................................
87
4-4 Microscope image of the two-flap supercapacitor device after the initial
fold and application of carbon paint ...............................
4-5
87
Microscope images of the two-flap supercapacitor device after complete
assembly viewed from an (a) angle and from the (b) top ...............
.88
4-6 Microscope image showing the side view of a folded, two-flap supercapacitor device. The bottom half is a reflection of the top half. It can be seen
that membrane separation distance is much greater on the pyramid side of
the device ................
4-7
...................................
89
Side profile illustration of the process flow for the fabrication, painting,
and folding of an one-flap supercapacitor device. (a) KOH is used to etch
small pyramid shapes into the silicon substrate. (b) Metal layer for the
hinges and various wiring is deposited via e-beam evaporation and patterned with wet etching. (c) SU-8 layer is spun on and patterned to serve as
the structural material. (d) XeF2 gas is used to isotropically etch away the
underlying silicon and release the single flap. (e) Carbon paint is manually
deposited on the gold electrode surface. (f) The single released flap is
folded ...................
4-8
...................................
90
Microscope images of the carbon painted electrode area in (a) two-flap and
(b) one-flap supercapacitor devices. The SU-8 wall helps confine the
carbon paint within the gold area in the one-flap device while some of the
carbon in the two-flap device is touching the adjacent wire .............
4-9
.91
Microscope image of the one-flap supercapacitor device after carbon paint
deposition. The released flap on the bottom needs to be folded over to
complete the assembly ..........................................
16
92
4-10
Microscope images of a flap folded over 1800. (a) no elastic-spring back is
demonstrated due to broken or almost-broken hinges. (b) Elastic springback is shown ................................................
93
4-11
SEM image of a single square flap on the origami supercapacitor ........
94
4-12
SEM image of the supercapacitor's electrode region before the deposition
95
of carbon paint. The array of pyramids help increase the surface area .....
4-13
SEM image of the spacing and alignment pyramids ...................
4-14
Top-down SEM image of square opening fitted over an alignment pyramid. Alignment error is around 1 pm ..............................
4-15
.96
96
Top-down SEM image of square opening fitted over an alignment pyramid. Alignment error is around 2 pm ..............................
97
4-16
SEM image of a folded, two-flap supercapacitor device ................
97
4-17
Illustration of the Lorentz force actuation concept ....................
98
4-18
Illustration of the Lorentz force actuation concept with a continuously
rotating magnetic field .........................................
4-19
99
Test setup for Lorentz force folding with continuous magnetic field rotation. The device to be tested is suspended in air with a rigid rod to allow
the horseshoe magnet to free rotate around it ........................
100
4-20
Close-image of the suspended device. The horseshoe magnet is not shown.
100
4-21
One-flap Lorentz force actuation device (a) before testing and (b) after
being melted ................................................
4-22
101
Illustration of the multi-layer folding process using Lorentz force actuation. If the magnetic field is rotated back and forth as shown in the figure
and the folded flaps latched sequentially as shown, multi-layered origami
devices could be batch-fabricated .................................
4-23
103
Microscope image of a 5-flap device that as popped up out of the substrate
upon release ..................................................
17
104
4-24
SEM images of an one-flap supercapacitor device that has popped up to an
angle of approximately 130 ......................................
...
105
4-25 SEM image of a 2 pm thick gold layer suspended on a silicon column..... .105
4-26
Edge region behavior of a tensile film attached to a substrate. (a) No tensile
stress is present in the thin film. (b) Tensile stress in the thin film causes
the edge plane to bend ..........................................
4-27
106
Illustration showing the effect of SU-8 shrinkage on the edge plane of the
gold hinge layer. A stress-free gold bar that is attached to such a plane will
be bent dow nw ards ............................................
4-28
106
Results of FEA showing the upward bending of a stress-free gold layer due
to tensile stress present in the SU-8 layer. The thin layer on the bottom is
gold, and the thick layer on top is SU-8 ............................
4-29
107
Microscope image of an one-flap supercapacitor with an integrated mechanical latching system. The edges of the devices are outlined in red for
clarity ...................
4-30
..................................
An one-flap supercapacitor device with two photoresist pads for adhesive
bonding before the reflow process .................................
4-31
108
109
Photoresist pads after the reflow process. (a) The photoresist pad on the
bottom layer has fully melted. (b) Only a small portion of the photoresist
pad on the top layer has melted ...................................
110
4-32 SEM image of the second-generation supercapacitor with etch holes and a
wide center hinge. The new elements have shifted the etch release point as
5-1
shown in the figure ...........................................
I I
Experimental setup used for the electrochemical testing of supercapacitors.
114
5-2 New supercapacitor assembly used during the second round of testing. The
silicon reservoir surrounds only the folded flaps ......................
5-3
116
Nyquist plot generated from the AC impedance measurement of supercapacitors with carbon electrodes ...................................
18
117
5-4 Nyquist plot generated from the AC impedance measurement of supercapacitors with carbon electrodes ...................................
5-5
Galvanostatic charge (I = 100 pA) of s supercapacitor with carbon electrod es .. .. .. . . . . .. .. . .. . . . .. ... . . . . . . . .. . . . . . . . . . . . . . .. .. . . . .
6-1
119
Results of FEA on different hinge designs [105]. (a) rectangular hinge. (b)
hinge with concave sidewalls. (c) hinge with convex sidewalls ..........
6-2
118
123
The use of the Nanostructured OrigamiTM process in 3D photonic crystal
fabrication. Standard nanofabrication techniques are used to create the
array of 2D photonic crystals which are subsequently folded to create the
3D structure [10] .............................................
19
123
20
List of Tables
2.1
61
Initial parameters of a single gold hinge ............................
61
2.2 Initial dimensions of a single SU-8................................
2.3
Estimated parameters for Lorentz force actuation .....................
2.4 Estimated parameters for strain mismatch induced actuation ...........
3.1
.62
.63
Approximate dimensions of the two-flap SU-8 device before and after the
release step ...................................................
75
3.2
Fabrication process for 25 pm thick layer of SU-8 2025 ................
75
3.3
Fabrication process for 15 um thick layer of SU-8 2015 ................
76
4.1
Parameters of the gold hinge .......................................
93
B. I
MTL process flow for first-generation supercapacitor devices ................
129
D. 1
MTL process flow for second-generation supercapacitor devices ..............
137
21
22
Chapter 1
Introduction
Without a doubt, tremendous technological advances made in the area of microfabrication, and more recently in nanofabrication, have literally changed the world. From airbag
sensors and atomic force microscopes (AFM) to very large scale integrated (VLSI)
systems and X-ray lithography, the fingerprints of this ever-growing technology can be
seen everywhere. In fact, almost all of the top 25 innovations of the past quarter-century,
as compiled by the Lemelson-MIT Program and CNN [1], have been made possible by
the advent of micro- and nanofabrication technology.
Bulk of the research effort for the past two decades has been in the area of twodimensional (2D), or planar, fabrication where all the features created are essentially flat.
In most applications, such as microprocessors, having 2D features is sufficient, and
current fabrication techniques are adequately quick, cost effective, and efficient in 2D
manufacturing. The semiconductor industry, in its unending quest to make things cheaper,
faster, and smaller, has contributed heavily to this field and has accelerated the development of incredibly powerful and highly efficient planar fabrication methods. Intel, for
example, can now manufacture Pentium chips with well over 1 billion transistors per chip
at a cost of less than 1/10,000th of a cent per transistor [2].
For the semiconductor industry, entering the nanotechnology era was a natural course
of action as physical limitations of Moore's Law were being challenged. Reducing size so
that more devices can fit in a given area isn't the only benefit of nanotechnology, however. Mechanical, electrical, optical, and chemical properties of materials can become
23
completely different when changes are made at the nanoscale. For example, carbon
nanotubes exhibit fantastic mechanical properties while nanoscale particles vastly increase the surface area of a material to enhance its chemical reactivity. Advances in
nanotechnology will soon enable novel applications in the fields of bio-sensing, computing, and energy conversion, among many others. Not surprisingly, research in nanofabrication technology is progressing at a feverish pace.
One major drawback of current methods in nanoscale manufacturing, however, is that
they are still designed primarily for planar fabrication. For applications requiring threedimensional (3D) structures with nanoscale features, current planar fabrication methods
face severe limitations.
Clearly, there is need for a new 3D nanomanufacturing procedure that would allow
nanoscale fabrication in non-planar configurations. For commercial viability, such a
process should also take advantage of existing semiconductor and microelectromechanical systems (MEMS) industry infrastructure and be compatible with current fabrication
techniques.
1.1
3D Nanomanufacturing
While no commercially available fabrication technique completely satisfies the criteria
for a viable 3D nanomanufacturing process, state-of-the-art micro- and nanofabrication
techniques have experienced remarkable progress in recent years and have addressed
some of the challenges associated with it.
1.1.1
Nanofabrication
Nanofabrication can be essentially categorized into two main approaches: "top-down"
and "bottom-up."
In general, top-down methods refer to building nanoscale features by out of larger
components (e.g. silicon wafer). Traditional photolithographic techniques would be
classified as top-down, although their nanopatterning abilities would be severely restricted due to the diffraction limit of light and the nonlinear properties of available
24
photoresists. Not surprisingly, most top-down nanofabrication techniques are akin to
more conventional fabrication techniques used by the semiconductor industry in integrated circuit (IC) manufacturing. These methods include optics-based processes that
work at shorter wavelengths such as electron-beam (e-beam) lithography, X-ray lithography, and extreme ultraviolet lithography (EUVL). Soft lithography, another top-down
nanofabrication method, refers to a set of completely different fabrication techniques
such as replica molding (REM), micro-contact printing (pCP), micromolding in capillaries (MIMIC), micro-transfer molding (uTM), solvent-assisted micromolding (SAMIM),
and near-field conformal photolithography using an elastomeric phase-shifting mask [3].
These procedures do away with conventional rigid photomasks and instead use a patterned elastomer to transfer patterns directly to the desired surface. Structures as small as
10 nm have been demonstrated using this technique. Lastly, dip-pen lithography uses an
AFM tip to "write" onto a substrate by directly transporting the desired molecules to the
substrate [4]. Minimum line widths and dot diameters of 15 nm have been successfully
demonstrated with this technique [5].
Bottom-up methods are inspired by biological processes and build up to the final
structure by self-organizing smaller components that are often at molecular or even
atomic scales. These methods are mainly chemistry-based and include, among many
others, nanowire superlattice structures [6], self-assembling peptides [7], DNA nanoconstruction [8], and self-assembled block copolymers [9].
1.1.2 Three-dimensional fabrication
Increasingly, more research effort is being directed to 3D fabrication. However, many of
these techniques are not suitable for commercial applications, and not all of them can be
scaled down to the submicrometer regime.
One type of 3D fabrication method uses multiple micro- or nanopatterned 2D layers to
create the final 3D structure. Although these devices are not truly 3D in the sense that
only the planar surfaces are patterned and very high aspect ratios are not easily attainable,
they are still useful in many applications, for instance, 3D photonic crystals. One way to
achieve such structures is through a layer-by-layer fabrication approach [10] in which
each successive layer is deposited and patterned using standard nanofabrication methods
25
(Figure 1-1). Because e-beam lithography is used to pattern the 2D layers, nanoscale
features are possible along the plane. Another way to create such multi-layered structures
is by physically stacking 2D layers on top of one another (Figure 1-2). For example,
Noda et al. stacked 0.7-um period semiconductor stripes with a precision of 30 nm using
an advanced wafer-fusion technique [11]. However, both of the processes mentioned
above are very complex and time-consuming compared to more standard planar fabrication methods. In addition, these techniques can only be used to create layer-by-layer 3D
structures that are essentially stacks of patterned 2D layers.
Figure 1-1: Cross-sectional SEM image of a 3D photonic crystal created using a lithographic
layer-by-layer approach. Seven functional layer can be seen [10].
4th
I St
2nd
3rd
,
,
1st 2nd
(b)
(a)
Figure 1-2: 3D photonic crystal structure created via highly precise stacking [11]. (a) Schematic
drawing of a four-layer structure. (b) SEM image of a two-layer structure.
26
In microstereolithography [12], complex 3D shapes (Figure 1-3) are formed by
stacking thin films of hardened, patterned polymer layer upon layer. The desired parts of
each polymer layer are hardened by either scanning point by point with a UV laser or by
using a photomask to pattern the whole layer at once. Either way, this process is very
time consuming, and fabrication of nanoscale structures is difficult due to poor lateral
resolution and relatively thick (>lpm) polymer layers. In addition, the choice of materials
that can be used with this method is very limited.
Figure 1-3: SEM image of a car fabricated using microstereolithography. The car is approximately 2 mm in length and around 3 hours to complete [13].
Three-dimensional holographic lithography [14] is well suited for fabrication of
periodic 3D structures, such as 3D photonic crystals (Figure 1-4). The 3D structure is
created by interference of four non-coplanar laser beams in a thick layer of photoresist.
Regions of the photoresist exposed by the 3D interference pattern become insoluble, and
the unexposed regions are dissolved away. By using a polymer such as SU-8, which has
intrinsically low absorption and can form very thick layers, holographic lithography can
generate tall 3D structures with sub-0.1 pm resolution [16]. However, this method can
only be applied to a limited selection of materials, and non-periodic 3D structures cannot
be created. Another problem with this technique is shrinkage, which can distort the 3D
structures and compromise their structural integrity. For example, 3D photonic crystals
with defects, such as waveguides, cavities, etc., cannot be created through holographic
lithography techniques alone.
27
Figure 1-4: SEM image of a 3D photonic crystal fabricated via holographic lithography. Four
non-coplanar laser beams were used to create a 3D interference pattern in a layer of SU-8 [15].
Multiphoton fabrication methods [17] can be used to create complex 3D shapes that
cannot be created with conventional lithographic techniques (Figure 1-5). The two-
photon absorption polymerization takes advantage of the fact that localized absorption of
photons can be achieved with a tightly focused laser beam. By scanning the focal point of
such a laser beam in a medium such as photoresist, intricate 3D structures can be formed.
Furthermore, features smaller than would be expected from the diffraction limit of the
light used can be created through a chemical nonlinearity in the patterned medium that
results in an intensity threshold for polymerization. Based on this technique, features as
small as 120 nm have been created with a 820-nm laser [18]. Unfortunately, point-bypoint scanning can be extremely slow especially for larger structures, and only a limited
selection of materials may be used with this method. Methods that combine two-photon
absorption polymerization with other techniques such as microtransfer molding [20] and
holographic lithography [21] help improve slow fabrication times.
28
(a)
(b)
Figure 1-5: SEM images of complex 3D shapes created with two-photon absorption polymerization. (a) A microbull with 150-nm minimum feature size. Fabrication time is approximately 3
hours [18] (b) Cross section of 3D photonic crystal made from SU-8 [19].
A bottom-up approach can be applied to 3D fabrication as well. For example, a
photonic bandgap crystal (Figure 1-6) has been fabricated by colloidal assembly of
microspheres [22]. While this process is relatively quick and simple, it is severely limited
in terms of attainable shapes, and non-period shapes are not possible.
(a)
(b)
Figure 1-6: SEM images of 3D structures formed by colloidal assembly of microspheres [22]. (a)
Face-centered cubic lattice synthetic opal template formed by self-organization of 855-nm
spheres. (b) Silicon photonic crystal formed by conformal filling of the opal template.
Using commercially available MEMS fabrication methods, 3D microstructures have
been created by folding polysilicon plates connected with micromachined hinges [23].
Because the hinges introduce various constraints, the 3D system is reduced to a single
degree-of-freedom. In a process that resembles children's pop-up books, complex 3D
29
microstructures are created by manually flipping a single plate (Figure 1-7). Although
assembly time is greatly reduced compared to a more traditional "flip up and lock" type
of design [24] where each plate raised out of the substrate and held in place by another
supporting plate, manual assembly is still required and becomes a bottleneck in batch
fabrication. In addition, nanoscale precision in the final 3D structure is difficult to
achieve due to inherent mechanical play in the micromachined hinges. Finally, the singlestep assembly technique of complex 3D microstructures can be applied to a limited
selection of 3D geometries as only specific types of 3D structures can be reduced to a
system with a single degree-of-freedom.
(b)
(a)
Figure 1-7: SEM images of 3D microstructures created with a single-step assembly technique
[24]. (a) Corner cube retroflector (CCR) after manual flipping of one plate. (b) Pop-up box that is
closed on all four sides.
1.2
Overview of Nanostructured OrigamiTM
process
The Nanostructured OrigamiTM 3D Fabrication and Assembly Process [25-27] is a
completely different approach to 3D nanofabrication. It is based on the Japanese art of
paper folding called origami. The key element of this innovative process is that the "3D"
part is essentially decoupled from the "nanofabrication" part. Consequently, many planar
30
(2D) nanofabrication techniques, some of which have been discussed above in Section
1.1.1, can be incorporated into this 3D fabrication scheme.
During the "nanofabrication" stage of the origami process (Figure 1-8a), conventional,
or perhaps not-so-conventional, planar fabrication techniques are used to create microand nanopatterned 2D membranes. These 2D membranes could be anything ranging from
standard IC and MEMS components to novel microfluidics and photonics systems.
During this 2D fabrication stage, the membrane is also patterned with creases, hinges,
and other elements that will allow it to be folded.
The patterned 2D membranes are folded into their final 3D configuration during the
"3D" stage of this process (Figure 1-8b, Figure 1-8c). During this part, 2D membranes
patterned in the first stage are automatically folded and aligned into a 3D geometry by
means of various actuation and alignment mechanisms discussed later in Section 2.1.
(a)
(b)
Figure 1-8: Conceptual drawings illustrating the Nanostructured Origami
(c)
TM
process. (a) During
the first stage of the process, planar fabrication methods are used to pattern a 2D membrane.
(b) Various actuation and alignment mechanisms are used to automatically fold the 2D membrane
into a 3D configuration. (c) The final nanopatterned 3D devices.
1.3
Electrochemical energy storage and conversion
devices
Microsystems enabled by advances in MEMS and IC fabrication technology still lack one
critical element: an efficient, integrated, microscale power supply. For example, Smart
Dust, developed at the Berkeley Sensor and Actuator Center, is a complete sen-
31
sor/communication system with a sensor, power supply, analog circuitry, bidirectional
optical communication, and a programmable microprocessor all integrated into a cubic
millimeter package [28]. However, because the device relies on a bulky, external power
supply (a hearing aid battery), complete miniaturization and integration remain challenging. In this and many other cases, the power supply is a bottleneck for further miniaturization and integration.
Not surprisingly, many have tried to take on the challenge of microscale power
integration. Some of the many approaches include microscale fuel cells [29], microturbines [30], microbatteries [3l]-[34], and solar cell arrays [35], just to mention a few.
However, complicated micromachining processes, incompatibility with conventional IC
fabrication processes, low capacity, poor performance, and high manufacturing cost are
just some of the associated problems that continue to impede successful integration and
implementation of microscale power systems.
Successful microscale power integration cannot occur without a reliable energy
storage device. Whether or not the microscale power supply can generate or capture
energy, an energy storage medium is required. According to Koeneman et al [36], an
electrochemical approach is the most efficient and feasible solution to the microscale
power storage problem due to its high energy density and ease of fabrication.
1.3.1
Advantages of 3D and nanoarchitecture
Full-size electrochemical energy storage and conversion devices, such as batteries,
supercapacitors, and fuel cells, are based mostly on a 3D geometry. For such devices, an
increase in surface area usually translates to an increase in performance; building in 3D
can increase the total area of reactive surfaces without increasing the areal footprint of the
device. The same is also true of electrochemical devices at the microscale. For a given
area, a 3D microbattery would be capable of much greater cell capacity compared to its
2D counterpart [37]. In addition, certain electrochemical devices, particularly some of the
more complex fuel cells, simply cannot be built in a 2D configuration.
Performance characteristics of electrochemical energy storage and conversion devices
can also be improved dramatically by the addition of nanoscale features [38]. For example, performance of lithium-ion batteries have been improved by adding nanostructured
32
electrodes [39]. Consequently, many companies and research institutions are working on
ways of improving electrochemical performance by using nanostructured materials and
surfaces. In a type of an electrochemical energy storage device known as a supercapacitor,
for instance, researchers are incorporating nanostructured manganese dioxide [40],
carbon nanotubes [41]-[44], and carbon nanofibres [45] into the electrode for increased
surface area and therefore increased capacitance.
Unfortunately, fabrication of 3D energy storage devices, especially those that can be
batch fabricated and integrated with existing devices and processes, is difficult. Furthermore, the incorporation of nanostructured surfaces to such electrochemical devices is
even more challenging. One patent [46] describe the microfabrication process for an
electrochemical supercapacitor, but the resulting structure is not truly 3D and cannot be
incorporated with nanoarchitecture.
Clearly, electrochemical energy storage and conversion devices can benefit greatly
from the Nanostructured OrigamiTM process, which provides the means of achieving both
a 3D configuration and nanoarchitecture. Not only will these devices exhibit an improvement in performance, the origami process will allow such devices to be fabricated at
a scale never before realized. Fabrication of micro-scale, electrochemical energy storage
devices that could be integrated with existing MEMS or complementary metal oxide
semiconductors (CMOS) processes, therefore, would prove highly useful. The Nanostructured OrigamiTI process could make this possible.
1.3.2
Electrochemical capacitor
In this thesis, we demonstrate the application of the Nanostructured OrigamiTM process to
the fabrication of a particular type of a electrochemical energy storage device called a
supercapacitor [47]. Also known as an electrochemical capacitor, the supercapacitor is a
type of a capacitor in which the energy is stored within an electrochemical double-layer,
or the Helmholtz Layer, at the electrode/electrolyte interface [48]. Figure 1-9 illustrates
the principle of a double-layer capacitor.
33
Edectrle, Separator
I
I
Elecrolyte, Active Layer
I
Cufrent Colector
Curret Colector
+
CitWon particles in
contact with an
' Woyteafm
Figure 1-9: Principle of a double-layer capacitor. Electrolytic solution spreads throughout the
porous carbon structure, and charge is accumulated at the resulting electrode/electrolyte interface
[48].
T"~"
ri~
fis
__
__
__
106
0f
CD
100
ii
61L
CELLS 1
10
It
0.01
I
0.05 0.1
I~I
.
I
5
I
I
10
II
50 100
500 1000
Specific Energy (Whlkg)
Figure 1-10: Ragone plot for different energy storage and conversion devices [48].
34
As Figure 1-10 suggests, supercapacitors can effectively bridge the gap between
conventional batteries and capacitors in terms of power and energy densities. The energy
storage mechanism in the supercapacitor involves no chemical changes and is therefore
completely reversible. The supercapacitor's almost unlimited cyclability and high power
density make it ideal for complementing batteries in many high demand applications.
Furthermore, due to the supercapacitor's extremely high specific capacitance, it is
emerging as a viable alternative to conventional electrostatic capacitors.
One main difference between the electrochemical capacitor and its electrostatic
counterpart is that the former requires an electrolytic solution. However, the principle of
operation remains generally the same as the capacitance value in both types of capacitors
is given by the well-known equation
C=k
"
d
(1.1)
where C is the capacitance, k is the dielectric constant, co is the permittivity of free space,
A is the electrode surface area, and d is the thickness of the dielectric layer. However,
surface area A is defined as the total surface area of the highly porous carbon structure,
and thickness d is defined as the separation distance between the electrode surface and
the ions. This ionic separation distance depends on the concentration of the electrolyte
and on the size of the ions and is typically in the order of 5 to 10 angstroms [48]. With
certain types of carbon exhibiting a specific surface area of over 2000 m2/g [47], it comes
as no surprise that supercapacitors based on the principle of electrochemical double-layer
capacitance demonstrate a specific capacitance that is orders of magnitude greater than
the electrostatic capacitors.
1.3.3 Advantages of origami fabrication for
supercapacitors
The origami method of fabrication offers many advantages for the supercapacitor. First of
all, the majority of supercapacitors discussed in literature are macroscale devices formed
35
by hand. They cannot be produced through batch-fabrication techniques and certainly
cannot be integrated into existing MEMS and IC devices as an on-chip power source.
Another key advantage of the origami fabrication method is that the 2D membranes can
be nanopattemed via a variety of nanofabrication techniques before being folded. The
highly flexible nature of the origami process allows almost any kind of nanostructures to
be incorporated into the electrochemical device. In addition, unlimited stacking ability in
the 3 rd dimension allows the completed device to maintain a very small area on the chip.
For example, as seen in Figure 1-1 1, a supercapacitor layout that would normally require
a large areal footprint can be folded to result in a compact, multi-layer, 3D device.
t
Device Design Flexibility
-.--
3rd Dimension
HHHF
1]Gld----------------------E: Gold----
-- --
-
--
--
Voltage and Current Outputs
*U8
AvVarious
Mater als
Figure 1-11: Drawing of a multi-layer supercapacitor with flexibility in voltage and current
outputs.
1.4 Thesis objectives
The key topics of this thesis are the origami SU-8 process and the fabrication and testing
of 3D nanostructured electrochemical energy storage devices that are created using the
Nanostructured OrigamiTM process. Accordingly, the thesis is divided into two major
sections.
In the first section, a new set of materials and designs for the origami process are
considered and, if found to be advantageous, incorporated. The design and fabrication
36
steps of the new set of devices will be carefully outlined, and the completed devices will
be presented and analyzed. Actuation, alignment, and latching of the origami segments
are some of the key issues that must be addressed in order for the origami process to
become commercially viable. Although these concerns will be dealt with, the main
emphasis will be placed on creating a large number of supercapacitor devices suitable for
testing. To this end, proven manual assembly methods will be used heavily to increase
yield and speed up the fabrication process. Also, certain design elements of the origami
devices will be modified in some cases solely to increase yield and simplify assembly.
Using the new origami process optimized for supercapacitor fabrication, functional
supercapacitors were created and tested. The detailed process of making test-ready
samples will be presented, and the resulting devices will be thoroughly tested and analyzed. Because the work presented in this thesis represents the first ever attempt at
creating such devices, we will be satisfied with device performance that approaches that
exhibited by full-scale, commercial devices.
1.5
Outline of thesis
This chapter mentioned the need for a new 3D nanomanufacturing method and introduced the Nanostructured OrigamiTM process as a possible solution. The advantages of
using this method in fabricating electrochemical capacitors was also discussed.
Chapter 2 describes the various functional requirements of origami fabricated devices
and how similar concerns are addressed in non-origami applications. Material selection
and other design parameters are also discussed.
Chapter 3 outline the fabrication process of new origami devices that are designed
with elements mentioned in Chapter 2.
Chapter 4 will present and analyze completed devices that are fabricated using the
process developed in Chapter 3. Analysis will include determining the effectiveness of
alignment, actuation, and latching mechanisms.
Chapter 5 describes the testing procedure for the electrochemical analysis of origami
fabricated supercapacitors. The experimental results are presented and discussed.
37
Chapter 6 is a final discussion of the work presented in the thesis. Future work and
other possible applications are also discussed.
38
Chapter 2
General Design Criteria for
Nanostructured OrigamiTM Devices
As discussed previously in Chapter 1, the Nanostructured OrigamiTM process can be used
to create a wide array of novel devices that take advantage of both the nanoscale features
and the 3D geometry provided by the process. Whether fabricating 3D integrated circuits,
3D photonic crystals, or 3D electrochemical devices, there are certain general functions
that the origami fabrication method must address. This chapter discusses such functional
requirements and also talk about specific design elements for the fabrication of the 3D
nanostructured electrochemical capacitor.
2.1
Functional requirements
Devices created via the Nanostructured OrigamiTM 3D Fabrication and Assembly Process
all share a number of common elements: rigid membrane, hinge, actuation mechanism,
alignment system, latching device, and a method of interconnection among the folded
layers. This section will present a literature review of available techniques that can
provide such functions and outline which, if any, can be applied to the Nanostructured
OrigamiTM process.
39
2.1.1
Rigid membrane and hinge
A rigid membrane, on which various micro- and nanoscale features are patterned, and a
hinge-like linking mechanism that connects such membranes are perhaps two of the most
crucial elements of devices created via the origami process. Although these two elements
should ideally be uncoupled (i.e. a change made in the design parameter of a specific
functional requirement shouldn't affect any of the other functional requirements) [49] as
to allow full independence in the design selection of the membrane and the hinge, limitations in the fabrication process and the need to minimize processing complexity and cost
mean that the designing of the membrane and the hinge must be considered together. In
fact, the design and material selection for many of the origami elements depend heavily
on one another for the same reason.
Although the Nanostructured OrigamiTM process is a very new concept, hingedmembrane-type structures have been created by other research groups in the past. For
most, the key area of interest in using hinged structures is to raise a horizontal membrane
into a vertical position. Applications include vertical spiral inductors with improved
performance and higher quality factor [50], vertical hot-wire anemometers for measuring
fluid velocity [51], standing mirrors for micro-optoelectromechanical systems (MOEMS)
[52], and even an electrostatically actuated insect wing for a microbot [53]. Three types
of hinges are generally reported in literature: a surface micromachined mechanical hinge,
a plastically deformable hinge, and a photoresist hinge.
In hinged structure designs that utilize a surface micromachined hinge, polysilicon
hinges link together rigid polysilicon membranes [54][55][24]. Because the multi-layer
surface micromachining process exclusively uses polysilicon as its structural material,
both the hinge and the rigid membrane are almost always made out of polysilicon. Figure
2-1 shows such a hinge made from a three-layer polysilicon surface micromachining
process (the Sandia SUMMiT process). Although the use of mechanical hinges in conjunction with rigid polysilicon membranes has been widely demonstrated and can be
realized with widely available commercial fabrication tools, there are several drawbacks
40
to this method in regard to the origami method of fabrication. First, the surface micromachining process for a mechanical hinge, although widely used in the MEMS community, still requires several structural and sacrificial layers and can be quite complicated.
Second, mechanical slack in the hinges cannot be completely eliminated once the sacrificial layers have been etched away. Such a slack may adversely affect the final geometry
of the completed device. Lastly, electrical connection through the hinges can be difficult
to achieve. For a device such as the supercapacitor where electrical connections must be
made to each folded segment, a different type of hinge and membrane design must be
considered.
Figure 2-1: SEM image of a surface micromachined substrate hinge holding down a horizontal
polysilicon flap on the silicon substrate. The hinge is comprised of two polysilicon layers [24].
Another type of hinged structures is based on plastically deformable hinges. In these
devices, the hinge is simply a slab of elastic material that connects two rigid membranes.
Because these type of hinges can be made of a conductive material such as aluminum,
electrical connection can be easily established. Also, a number of different materials may
be used to construct the stiff membrane structures.
Vaccaro et al. have created micromirrors and other similar 3D devices using a strained
hinge layer [52][56][57]. In these devices, a heteroepitaxial SiGe/Si strained layer is
deposited over the entire substrate and topped with an epitaxially grown rigid structural
layer (e.g. silicon). The overall shape of the device is created by etching through all three
layers (structural, strained, and sacrificial) at selected regions, and the flexible regions
that act as hinges are defined at desired locations (i.e. folding axes) by etching away only
41
the top structural layer (Figure 2-2). Section 2.1.2 will discuss how the strained SiGe/Si
layer is also used to allow automated folding of these devices.
4
Strained hinge layer 4
Sacrificial layer 4
Rigid structural layer
Figure 2-2: Diagram of the strained hinge device before release. Etching all three layers defines
the shape of the device while etching only the top layer creates flexible bending regions [52].
In a similar technique commonly known as plastic deformation magnetic assembly
(PDMA) [50][51][58], a layer of elastic material such as gold is coated with a thick, rigid
Permalloy layer. The Permalloy layer is subsequently patterned to define the inflexible
regions. Figure 2-3 shows the bending of rigid Permalloy bars at the gold plastic bending
regions. Because the deformable bending region is electrically conductive, it can provide
electrical connection to the flaps. The Permalloy layer, in addition to providing structural
support, enables magnetic actuation of released membranes. The actuation aspect of the
PDMA method will be discussed shortly in Section 2.1.2.
(a)
(b)
Figure 2-3: SEM images of PDMA devices [50]. (a) Permalloy defines a rigid layer on top of the
flexible gold layer. (b) Devices are bent at the god plastic bending region.
42
The final type of linkage mechanism for rigid segments is the photoresist hinge [59].
The photoresist acts as a bridge between two rigid segments and allows movement only
in its melted state. Without additional mechanical support structures, however, the
photoresist alone does not provide very much precision during the folding process. When
used with other mechanical constraints, as will be discussed in Section 2.1.2, the photoresist hinge may prove to be an effective actuation mechanism. Figure 2-4 shows two
polysilicon flaps that are connected with photoresist. The flaps can technically be made
with any type of material, but polysilicon is usually used due to its ease of fabrication and
high stiffness.
Figure 2-4: SEM images of two polysilicon flaps connected with a photoresist hinge [59].
Of the various types of hinged structures discussed above, the plastically deformable
metallic hinge appears to be the most appropriate choice for origami nanofabrication of
3D electrochemical energy storage devices. A simplified fabrication process, lack of
mechanical slack in the hinges, and electrical connectivity across the gaps are some of the
main advantages of this process. Material selection for the hinge and membrane will be
discussed in Section 2.2.
43
2.1.2 Actuation
In order to avoid painstaking manual assembly of 3D devices [23], the Nanostructured
OrigamiTM process must make use of self-actuation mechanisms. Such actuation mechanisms, at least those that can be applied to the Nanostructured OrigamiTM process, can be
largely categorized into four different types: mechanical manipulation, surface tension,
strain mismatch, and magnetic force. While many other actuation methods exist, these
four actuation schemes are most widely used for out-of-plane actuation application.
Additionally, just as the design requirements of the hinge and the membrane depend
heavily on each other, the choice of actuation mechanism also depends a lot on the type
of hinge and membrane used.
The external mechanical manipulation method uses a combination of linear actuation
devices and mechanical linkages to cause out-of-plane motion. Used frequently in
MOEMS applications, these types of actuation devices are typically used to tilt micromirrors for beam steering in optical switching and laser scanning. Types of linear actuators
that can be used with this method include linear thermal actuators [60], comb drive
actuators [60], electrostatic microengines [60], linear microvibromotors [62], and vertical
thermal actuators [63]. Figure 2-5 shows a micromirror that has been raised out of the
substrate using a comb drive actuator and a complex driving mechanism. Although this
method allows a relatively precise control of large, out-of-plane motions, it is useful in
only a limited number of applications due to the excessive amount of area that the
actuators and the mechanical linkages require. For most origami applications where we
are interested in only the final 3D device, it would be unwise to set aside so much space
for an actuation device that will be used only once during the initial folding stage. In
addition, structural materials other than polysilicon will be difficult to incorporate since
all of the commonly used linear actuators are based on surface micromachining of
polysilicon. However, this method may prove useful in dynamic origami systems where
continuous reconfiguration is necessary.
44
Figure 2-5: SEM image of a micromirror raised via comb drive actuators and a complex mechanical driving mechanism [61].
Surface tension-powered assembly methods [59] take advantage of the fact that forces
due to weight scale with volume while forces due to surface tension of liquids scale with
length. Therefore, surface tension forces can be dominating in the domain of microstructures. In surface tension methods, meltable materials such as solder or photoresist are
deposited at the folding regions of the membrane. Upon heating, these materials melt and
deform to minimize surface energy. The surface energy of the liquid is not at its mini-
mum when the flap is in the horizontal position; the flap will rotate to an angle determined by the volume of the liquid. The drawing shown in Figure 2-6 shows a flap being
folded to 900 due to surface tension forces. One major drawback of this method is that
high angular precision cannot be achieved without additional angular placement mechanisms such as a mechanical limiter [64]. Another disadvantage is that elastic hinges,
which would enable electrical connectivity across membrane gaps as previously mentioned in Section 2.1.1, cannot be used since surface tension forces (which scales with
length) cannot easily overcome elastic forces (which scales with area) as it can gravitational forces (which scales with volume) [65].
45
(a)
(b)
(c)
Figure 2-6: Diagram of a flap being folded to 900 due to surface tension forces [59].
(a) A
meltable material such as solder or photoresist is deposited at the folding crease. (b) The deposited material is melted. (c) Surface energy minimization of the melted material results in a
deformation of the material and a rotation of the flap.
Actuation methods based on strain mismatch can be explained by a simple bimetallic
strip. When a bilayer strip of metals with different thermal expansion coefficients under-
goes a temperature change, the strip will curl to compensate for the strain mismatch at the
layer interface (Figure 2-7). Researchers have incorporated a number of different techniques to induce such a strain mismatch and use it to create curling or bending structures.
Bimorph piezoelectric actuators use materials such as zinc oxide (ZnO) and lead zircon-
ate titanate (PZT) that change dimensions when a voltage is applied [66]. If one layer of a
bimorph device undergoes an expansion or a contraction, the device will naturally bend.
Conducting polymers such as polypyrrole (PPy), which can undergo a very large volume
change, can also be used in such a manner [67]. Vaccaro et al. use a pair of latticemismatched epitaxial layers [52] to obtain the strain mismatch and achieve curling. In a
slightly different approach, researchers at Palo Alto Research Center (PARC) have
controllably curled a single layer of molybdenum-chromium (MoCr) by changing the
ambient pressure during film deposition and thus inducing a stress gradient in the film
[68]. Figure 2-8 shows an out-of-plane inductor that was fabricated by engineering the
stress in a MoCr layer. In origami fabrication, strain-mismatched bimorph devices could
serve as self-curling hinges. However, tight bending radii will be difficult to achieve with
the small amount of strain that is typically exhibited by materials used commonly in
micro- and nanofabrication. As a result, multi-layer type devices with very small layer-to-
layer spacing may need to utilize a combination of this and other actuation methods. For
46
example, the strain mismatch method can be used to get the membranes in their approximate positions after which a different actuation system completes the assembly by
positioning the membranes to their final positions.
4.
i
Figure 2-7: Diagram illustrating the idea of bimorph actuation. When the top (black) layer shrinks
in volume, the entire structured bends up to compensate for the strain mismatch [69].
Figure 2-8: SEM image of a self-assembled, out-of-plane inductor created with a stressengineered MoCr layer [70].
Finally, a magnetic field generated externally [50][51][55] or on-chip [71] can be used
to induce actuation. In a PDMA process, a magnetic material such as Permalloy is
deposited on a flexible membrane [50]. When an external magnetic field is applied, the
magnetic material becomes magnetized in the applied magnetic field, and the planar flap
is bent out of the substrate due to the torque generated (Figure 2-9). Another type of
47
magnetic actuation is the Lorentz force actuation method [72]. Lorentz force acting on a
conductive strip of length L can be modeled by the equation
F =- L I
x
B
(2.1)
where F is the generated force, I is the applied current, x is the vector cross product, and
B is the magnetic flux density of the externally applied magnetic field. Under this method,
the magnitude of the actuation force can be controlled by adjusting the applied current,
and its direction can be changed by reorienting the external magnetic field. Because both
of the magnetic actuation methods are based on elastic hinges, however, final precise
positioning of folded flaps will require additional placement and latching mechanisms.
Micro flap
r - substrate
''
A
Flexible
region
t t t t t t t
Hext
Figure 2-9: Illustration of the PDMA process [50].
F=iBL
Pad(Fixed on the Base)
Hin e
b
B
Structure
H
M
h
Figure 2- 10: Illustration of the Lorentz force actuation method [721.
48
A truly effective origami device may need to use several different actuation methods
to satisfy all of its folding requirements. Nevertheless, the strain mismatch and Lorentz
force actuation methods seem most suitable for origami nanofabrication of supercapacitors for several reasons. First, both methods utilize elastic linkage mechanisms that can
enable electrical connectivity when made from a conductive material. Second, neither
methods require very much extra space for the integration of actuation components.
Furthermore, the strain mismatch method requires absolutely no additional elements for
actuation (e.g. no need for manual assembly, electrical power, heat, magnetic field, etc.),
and by controlling the current, the Lorentz force method allows precise manipulation
over the actuation of individual flaps, required in complex sequential folding of complicated 3D geometries.
2.1.3
Alignment
Precise alignment among folded membranes will be crucial in devices such as 3D
photonic crystals where layer-to-layer alignment precision will affect device performance.
Also, such precision will be required in devices where accurate layer-to-layer connections
must be made. Of course, if the origami membranes are connected via perfect hinges that
allow only pure rotation, the folding pieces will be perfectly constrained in motion, and
further alignment mechanisms may not be needed. However, the hinges used, no matter
how precisely fabricated, will inevitably allow some undesired movement and will lead
to membrane misalignment. While much work has been done regarding high precision
wafer alignment techniques, not many widely available alignment methods can be
applied to folded membranes used in the Nanostructured OrigamiTM process.
One type of alignment technique uses mechanical couples to passively force layers
into alignment. Aoki et al. created a 3D photonic crystal by vertically stacking 2D
photonic crystal plates. Precise plate alignment throughout the structure was induced by
the mechanical coupling between polystyrene microspheres and precisely etched holes
[73]. Slocum et al. fabricated mating concave and convex elements using anisotropic
KOH etching and deep reactive ion etching [74] as seen in Figure 2-11.
49
(a)
(b)
Figure 2-11: SEM images of (a) convex and (b) concave elements. The two features mechanically
couple to allow passive wafer alignment [74].
Surface tension forces can also be exploited for alignment. Since capillary forces scale
with length, it can be dominant compared to other forces at the scale of origami devices
and can be an effective alignment tool. Srinivasan et al. have reported alignment preci-
sion of less than 0.2 pim for binding microscopic parts on a patterned substrate [75]. This
was achieved by photolithographically defining the microscopic parts and the binding
sites with complementary shapes of hydrophobic self-assembled monolayers. Shape
matching, and thus alignment, takes place due to the minimization of the interfacial free
energy of the system [76]. A similar technique based on capillary forces was also used for
high precision wafer-level alignment [77].
For applications requiring highly precise membrane alignment, passive alignment
techniques may not be sufficient. In fabrication of 3D photonic crystals, for example,
Noda et al. have achieved 30 nm layer-to-layer precision using an advanced, laser beam
assisted wafer-fusion technique [11]. However, the micron-level of alignment provided
by mechanical coupling and capillary force based methods will be adequate for supercapacitors where extremely precise membrane alignment may not be necessary. Because
mechanical alignment features can be readily integrated into the supercapacitor device
with no further chemical treatment, which may damage the electrode material of the
supercapacitor, an alignment scheme based on mechanical coupling will be used in the
origami fabrication of electrochemical capacitors.
50
2.1.4 Latching
Latching mechanisms are required to permanently lock folded origami pieces in place.
Much work has been done on wafer-level and chip-level bonding using solder and
different kinds of adhesives [78] for packaging and hybrid systems (e.g. IC and MEMS
devices on the same chip) applications; it may be possible to utilize adhesive materials
such as UV curable epoxy or reflowed photoresist to bond origami membranes. Mechanical latching is another possibility. Kolesar et al. have created hinged polysilicon
flaps with microrivets that squeeze into square openings and lock into place [79]. Using
standard micromachining techniques, Han et al. have created a "micromechanical Velcro" [80] that can be interlocked when pressed together (Figure 2-12). Another interesting approach takes advantage of various surface forces to permanently bond microstructures to the substrate [81]. By controlling stiction, which can be caused by capillary,
electrostatic, or van der Waals forces, permanent adhesion of microstructures can be
achieved.
17
Figure 2-12: Schematic cross section of micromechanical Velcro structures. When two surfaces
covered with these structures are pressed together, the tabs deform and spring back to create an
interlocked structure [80].
Stiction usually involves a thin film and a flat substrate with very smooth surfaces,
both of which our electrochemical device may or may not possess. In addition, it is very
difficult to control exactly when and where stiction will occur. For controllable locking of
51
folded membranes in the origami supercapacitor, adhesive and mechanical latching-based
methods seem most appropriate.
2.1.5
Interconnection
In addition to in-plane electrical connectivity that can be provided by conductive, elastic
hinges as mentioned in Section 2.1.1, vertical connections across folded layers are needed
to create a true 3D electrical network. Such a configuration may be necessary in applications such as 3D IC devices where massive amounts of interconnections must be made
between layers. While making such connections will not be crucial to the functionality
and performance of origami fabricated electrochemical devices, it is definitely an area of
research that should be undertaken in the near future. A flip-chip type solder bonding, use
of conductive adhesives, and optical communication via vertical cavity surface emitting
lasers (VCSELs) and photodiodes are just a few of the possibilities for establishing
vertical connection across the layers.
2.2 Material selection
An electrochemical capacitor fabricated via the Nanostructured OrigamiTM process will
consist of three main components: rigid membrane, hinge material, and electrode material.
As discussed previously, the hinges should be made of a conductive, elastic material that
enable folding as well as electrical connectivity across membranes. In addition to the
standard origami fabrication process, an electrode material will be added to the final
device in order to create a functional supercapacitor.
2.2.1
Membrane
The first generation of origami devices [26] were created on silicon on insulator (SOI)
wafers with the 10 pm thick silicon device layer acting as the origami membrane. While
the rigid silicon layer served as a good structural material and could be easily patterned
using standard micro- and nanofabrication techniques such as deep reactive ion etching
(DRIE) and electron beam (e-beam) lithography, the required wet release step for re-
52
moval of the sacrificial oxide layer caused stiction problems. In addition, the high cost of
SOI wafers left very little room for fabrication mistakes.
Octafunctional epoxidized novalac, or SU-8, is an epoxy-based, negative photoresist
based on the EPON SU-8 epoxy resin [82]-[84]. Because SU-8 is a type of a photoepoxy,
it can become polymerized through a cationic photopolymerization process [85]. During
this process, Lewis acids that are generated during the UV illumination stage induce
crosslinking of the epoxy resin into a 3D network structure. Polymerized SU-8 is a highly
crosslinked structure with very good mechanical properties. As a result, SU-8 has become
widely used as the structural material in many MEMS applications, and it could certainly
be used as the membrane material in fabricating Nanostructured OrigamiTM devices.
There are many other reasons for the increasing use of SU-8 in MEMS applications.
First, high aspect ratio (>20) structures with almost vertical sidewalls can be created with
thicknesses ranging from 0.5 pm to over 2 mm [86]. This can be attributed to the fact that
SU-8 has a very low optical absorption in the UV range where it is most sensitive;
exposure does across the thickness will be relatively uniform. Second, SU-8 is naturally
an excellent insulator, making it ideal for use in applications such as electrochemical
capacitors where the membrane layer must provide sufficient electrical isolation. Third,
SU-8 exhibits excellent chemical and temperature resistance. In fact, the highly robust
nature of SU-8 has caused processing problems for many researchers who simply could
not find ways of removing it in its highly crosslinked state. Fourth, the thickness of the
SU-8 layer can be easily varied by changing the spinning speed during coating. Such
flexibility in membrane thickness would not be possible when using, for example, SOI
wafers with a predetermined device layer thickness. Fifth, the bottom surface of SU-8 can
be patterned by depositing SU-8 over a mold, as in soft lithography. This would be
extremely useful in creating origami membranes with patterned top and bottom surfaces.
Finally, and perhaps most importantly, SU-8 processing is extremely easy and low-cost
compared to other fabrication methods for MEMS structural materials; it is simply spun
on, exposed, and developed like any other photoresist. With so many great advantages, it
is no wonder why SU-8 is gaining such popularity in the MEMS community as a structural material.
53
Unfortunately, there are some problems associated with SU-8. As mentioned previously, SU-8 is notoriously difficult to remove and almost impossible to pattern once it's
been initially developed and polymerized. In addition, SU-8, as with other crosslinked
polymers, exhibits shrinkage in volume upon polymerization. Guerin et al. reported a
shrinkage factor of 7.5% [87]. Of course, this shrinkage factor increases as the SU-8
becomes more highly crosslinked. Such shrinkage can have an adverse effect on the
devices, such as photonic crystals, where exact dimensions are required. The shrinkage
effect can also lead to a devastating result in the origami devices, as will be discussed in
Section 4.1.1. To compensate for SU-8 shrinkage, Rumpf et al. had to include the polymer shrinkage factor in the design of their SU-8 photonic crystal [88]. For the origami
devices, such shrinkage will have to be minimized or compensated for as well.
Despite some of the problems associated with SU-8, it was chosen as the membrane
material for the origami supercapacitor. The low cost and ease of fabrication, excellent
electrical isolation, chemical resistance, variable membrane thickness, and a bottom
surface that could be patterned via molding techniques all contributed to its selection.
2.2.2 Hinge material
As mentioned previously, the origami supercapacitor will use conductive, elastic hinges
to link together the various folding segments. Therefore, it is crucial that a material with
excellent mechanical as well as electrical properties be chosen for this role. The material
used for the hinge must meet four basic requirements: availability, conductivity, ductility,
and elasticity.
The first two requirements, availability and conductivity, do not need much further
explanation. The Microsystems Technologies Laboratories (MTL), where all the fabrication processes are conducted, has a limited number of materials that are available to the
user, and the material chosen must obviously be within the capabilities of the fabrication
facility. The material chosen must also exhibit good conductivity since the hinges are
used to establish electrical connections across the folds as well.
Ductility refers to the material's ability to withstand large amounts of plastic deformation without failure. For a hinge used in the origami device, such ductility is required
since bending to angles up to 1800 without breaking will require a large deformation. In
54
addition, SU-8 shrinkage discussed later in Section 4.1.1 will require the hinges to be
able to withstand a very large strain without breaking. Fortunately, fracture strains of
greater than I% have been reported for thin film metals such as gold [89], silver [90], and
copper [90].
High elasticity of the hinge material is required in order to reduce spring-back of the
hinge after bending. If the spring-back effect could be minimized, the folded membranes
will be more likely to stay in their folded positions after the actuation force is removed.
Spring-back occurs because sections of the bent material remain in the elastic region.
Since spring-back is proportional to the amount of yield strain, decreasing the yield stress
o., and/or increasing the Young's modulus will help reduce the effect.
Only a handful of materials available at the MTL meet these requirements. After
further eliminating materials such as chromium, which has a very high residual stress,
gold and copper were determined to be the most ideal choices. Both materials offer high
conductivity, high ductility, and little spring-back. Furthermore, both are widely processed materials at the MTL. While neither one appears to offer a clear advantage over the
other, gold was chosen because it is a more commonly used material in the MTL. It was
also reported previously [26] that a sacrificial copper seed layer would be required in a
PDMA based actuation scheme using electroplated Permalloy. Although this type of
actuation was not used in making the electrochemical capacitor, the use of copper was
avoided nonetheless for possible future inclusion of PDMA.
2.2.3
Electrode material
As mentioned in Chapter 1, electrochemical capacitors are different in principle from
standard electrostatic capacitors and require that the electrode material be chosen carefully. The three types of electrodes used in supercapacitors are carbon, metal oxides, and
conducting polymers.
Carbon, in its various forms, is used most frequently as the electrode material of
electrochemical capacitors due to its low cost, high surface area, availability, and established production technologies [48]. With certain forms of carbon exhibiting specific
surface areas greater than 2000 m2/g, very high specific capacitances can be achieved
using carbon. One main drawback of using carbon based electrodes is that depositing
55
carbon is not a standard fabrication process. Pyrolysis of photoresists has been reported
as a possible method of fabricating carbon structures [91], but required temperatures of
over 1000"C exclude it as a possibility from many temperature sensitive processes.
Metal oxides such as RuG 2 can also be used as the electrode material in supercapacitors, although most of the capacitance arising from such devices are pseudo-faradaic in
origin [92]. The extremely high price of metal oxides such as RuG 2 and their unavailability in the MTL excluded it from being used in the fabrication of the supercapacitor.
Furthermore, these capacitor materials can only be used with aqueous electrolytes and
may not be suitable for many applications.
Electrochemical capacitors based on conducting polymers such as polypyrrole also
derive most of the capacitance from pseudo-faradaic reactions [92]. However, long-term
instability due to swelling and shrinking of such electroactive polymers prevent their use
in many applications.
A form of carbon was chosen as the electrode material for the origami supercapacitors.
The wide availability and low cost of carbon contributed mainly to its selection. Although
carbon deposition is not a common clean room procedure, laser and chemical vapor
deposition methods may prove to be viable in batch fabrication applications. Detailed
composition and deposition method of the carbon electrode used will be discussed in
Section 3.2.4.
2.3
Hinge design
As mentioned in previous sections, gold beams will serve as conductive, elastic hinges
that link together the various SU-8 segments. In order for the gold hinge to function
properly, its dimensions must be carefully chosen. The three parameters of length (f),
width (w), and thickness (t) are shown in Figure 2-13. For the sake of analysis, gold will
be modeled as an elastic, perfectly plastic material, which exhibits fully plastic behavior
when loaded beyond yielding (Figure 2-14(a)).
56
Drawing showing the parameters 1, w, and t of the gold hinge that connects two
SU-8 segments. (Note: In the actual device, the SU-8 layer is above the gold layer, not the other
Figure 2-13:
way aroundas shown in the illustration.)
CIY
GC
E, strain
E, strain
(b)
(a)
Figure 2-14:
Stress-strain curves for (a) an ideal elastic, perfectly plastic material and (b) a
ductile material that exhibits necking behavior.
2.3.1
Failure analysis
The length of the hinge has little effect on its mechanical behavior. Therefore, the
length is chosen solely to satisfy geometric requirements. For example, when the hinge is
folded to 1800, its length should determine the separation distance between the folded
layers. If the hinge bends into a semicircular shape, as it was assumed in the design
process, the required length of the hinge will be simply wrd / 2 where d is the separation
distance between the two layers.
57
The width and the thickness of the hinge will determine whether or not it will fail
during the fabrication process. According to the maximum normal stress failure
criterion,
failure will occur when the largest principal normal stress equals the material's ultimate
tensile strength q-,. Due to necking behavior, however, ductile materials may not actually
fail when a,, is reached, but rather after further plastic strain at a lower engineering
fracture strength q1f (Figure 2-14(b)). Failure during fabrication will occur because, as
mentioned before, SU-8 shrinks when it becomes crosslinked. For most MEMS applications where SU-8 is left on the substrate, shrinkage causes only a reduction in final
thickness since the substrate restricts lateral motion. However, in our case, SU-8 is
released from the substrate, and a reduction in lateral dimensions will also occur. Because
the SU-8 segments are linked together with gold hinges, the shrinkage effect experienced
during the release process can lead to hinge failure. Figure 2-15 illustrates how the
shrinkage effect can lead to broken hinges.
Gold Hinge
SU-8 -
I
Si Substrate
Hinge Failure
Figure 2-15:
The series of drawings show what happens to the hinge during the release process.
As the silicon below the device is progressively etched away, the lateral shrinkage of the SU-8
causes the hinges to be stretched.
58
The stress on the hinges can be decreased by either reducing the axial load or increasing the cross-sectional area. Since the amount of axial load is related to the degree of SU8 crosslinking, the only thing that can be done from the hinge design point-of-view is to
increase cross-sectional area by increasing the thickness and/or width of the hinge.
Accordingly, the thickness and width of the hinges were initially set to 1.5 ptm and 50 ptm,
respectively. The thickness of 1.5 pm was decided upon for practical reasons as the ebeam evaporator used in the MTL for gold film deposition was not often used for film
thicknesses greater than 1 ptm. The width of 50 tm was thought to be the optimal hinge
width that would be wide enough to provide sufficient structural integrity and electrical
conductivity while being narrow enough to allow adequate wiring density across membrane gaps.
2.3.2
Lorentz force actuation
It was previously stated in Section 2.1.2 that the Lorentz force actuation method would be
an appropriate actuation mechanism for folding. Consequently, we must make sure that
Lorentz force, given earlier by equation (2.1), can overcome the elastic forces of the
hinges in order to successfully fold the membranes.
Normal stresses for linear-elastic bending of a beam with an applied moment M is
given by the equation
MY
I-
(2.2)
where I. is the moment of inertia of the cross-sectional area about the netural axis, and the
value y is the distance from the neutral axis. When a bending moment is applied to a
beam with a rectangular cross-section, there will be a linear distribution of stress a,
across the thickness of the beam as shown in Figure 2-16(a). Plastic deformation of the
beam will start to occur at the outer edges of the beam when qX becomes equal to the
yield stress oy, as shown in Figure 2-16(b). The theoretical stress distribution diagram for
fully plastic yielding of a rectangular beam is shown in Figure 2-16(c).
59
"y
Ay
Ay
t
t
2
2
2
2
2
2
t
--
(c)
(b)
(a)
Figure 2-16: Stress distribution diagrams for the bending of an elastic, perfectly plastic material.
(a) Fully elastic behavior. (b) After onset of plastic deformation. (c) Fully plastic deformation.
From Figure 2-16(c), we can derive the maximum moment required to fully plastically
deform a beam. Since the bending moment M of a beam is related to stress a- by the
general equation
(2.3)
M= foydA,
the maximum moment, M,e, required to bend a hinge of thickness t is given by
Mreq
f
/2
(2.4)
o7wdy
wtcro
Mre
(2.5)
' .
4
Using Equation (2.5) with the parameters of the gold hinge listed in Table 2.1, the
required maximum bending moment of Meq= 5.8
Since
the
Mreq,T =
initial
design
calls
for
x
10~9 Nm is obtained for each hinge.
four hinges,
a
total
bending
moment
of
2.3 x 10-8 Nm will be required. This value serves as an upper limit estimate for
the torque needed in order to fold the membranes.
60
Table 2.1: Initial parameters of a single gold hinge.
Parameter
Value
t
1.5 pm
w
50 pm
I-y
206 MPa [50]
Forces due to gravity should have a minimal effect on devices at this scale. A SU-8
flap with the dimensions listed in Table 2.2 and a density of 1.2 g/cm 3 [93] will have a
mass of approximately 1.7 x 10- Kg. This results in a moment about the hinge of
Mg = (1.7 x 10-8 Kg ) x (9.8 m/s 2) x (750 tm / 2) = 6.2 x 10-" Nm. This value is approximately three orders of magnitude less than the required bending moment for the four
hinges and will subsequently be ignored in further calculations.
Table 2.2: Initial dimensions of a single SU-8 flap.
Dimension
Value
length
750 pim
width
750 pim
thickness
25 ptm
The value of the Lorentz force can be calculated using Equation (2.1) that was given
earlier. The value for the current i is estimated for now using the "rule of thumb of 5
mA/pm2 [26]." This rule provides a crude estimate for the highest current that can be run
safely through a wire with a given cross-sectional area. Given the gold hinge's crosssectional area of 75 tm 2 , the generalized rule states that a current of 375 mA should not
melt the gold hinges. Section 2.3.4 will provide a slightly more detailed derivation of the
allowable current. The value for the magnetic flux density B was given by the manufacturer of the magnet that was used throughout the experiment. Subsequent testing of the
magnet with a magnetometer (from AlphaLab, Inc.) verified this value.
61
The maximum torque generated by Lorentz force actuation is given by the equation
(2.5)
Mmatg = L 2iB.
Using Equation (2.5) with the parameters listed in Table 2.3, a maximum applied moment
of Mmag = 2.1 x 10-8 Nm is obtained. This value is very close to MAeq,T that was calculated earlier and should be just enough to bend the hinges given a slightly higher current.
Table 2.3: Estimated parameters for Lorentz force actuation.
2.3.3
Parameter
Value
L
750 pm
i
375 mA
B
0.1 T
Strain mismatch considerations
Strain mismatch actuation is another type of folding mechanism that could be used to fold
the origami pieces without requiring manual assembly. The radius of curvature for a
strain-mismatched bilayer film can be obtained through beam bending analysis and has
been widely reported in literature. Most recently, Arora et al. [94] have reported that
1
E t
+
E t
p= -=
K
6E
+2EE tIt 2 (2t' +2t|+ 3tit 2 )
I
tIt 2 (t 1 +t
\
(2.6)
2 ) .8Ctr
where p is the radius of curvature, El is the elastic modulus of the bottom layer, E? is the
elastic modulus of the bottom layer, t, is the thickness of the bottom layer, t? is the
thickness of the top layer, and c',,, is the initial strain mismatch due to residual stress in
the top layer. The bottom and top layers would be gold and chromium, respectively, in
62
our device. A hinge of length
p = (1800
x 'hinge)
'hinge
would require a radius of curvature of
/ (o x r) to curl by Oo degrees. The plot in Figure 2-17, generated using
Equation (2.6) and the parameters given in Table 2.4, shows the bending angle as a
function of chromium layer thickness. It shows that approximately half a micron of
chromium deposited on top of the gold layer will cause the structure to curl by approximately 450.
Figure 2-17: Plot of bending angle vs. chromium thickness given the parameters in Table 2.4.
Table 2.4: Estimated parameters for strain mismatch induced actuation.
Parameter
Value
Ei
79.4 GPa [50]
E2
170 GPa [95]
tj
1.5um
Cgtoe
0.012 [94]
1hinge
100 pm
63
2.4 Pyramid structures
In Section 2.1.3, it was mentioned that a passive alignment system based on mechanical
coupling would be incorporated into the origami devices. This section will describe how
KOH-etched pyramids can help achieve such alignment. In addition, this section will
show how a 2D array of small KOH-etched pyramids can help increase electrochemical
performance of an origami fabricated supercapacitor.
2.4.1
Spacing and alignment
Anisotropic etching of silicon using potassium hydroxide (KOH) can be used to create
structures with very precise dimensions. For example, since the {I I} planes in a (100)
oriented silicon wafer are etched very slowly compared to the
{100}
planes, an inverted
pyramidal pit with a highly precise height can be created by allowing the
{ 11 } planes to
intersect and effectively self-terminate the etching process (Figure 2-18).
[1001
Masking layer
I
-
~12
(a)
111]
(b)
--
(c)
Figure 2-18: Fabrication of an inverted pyramidal pit using KOH etching. (a) The masking layer
is patterned to expose the silicon surface. (b) Etching in the [100] direction takes place very
rapidly while etching very slowly in the [111] direction. (c) Once the {111} planes meet, the
etching process is effectively self-terminated as only the slow-etching {111 planes remain.
By using a molding-type process, the highly precise pyramidal cavities can be used to
create protruding square pyramid shapes on the SU-8 membrane. Figure 2-19 and Figure
2-20 show how such pyramids could be used to achieve improved spacing and alignment
precision in the origami devices.
64
(a)
(b)
Figure 2-19: Conceptual drawings illustrating how pyramid structures could be used to improve
spacing and alignment. (a) The top flap is folded over and brought into contact with the bottom
flap. (b) Corresponding square openings on the top flap fit tightly over the pyramids on the
bottom layer and insure correct spacing and alignment between the two membranes.
Figure 2-20:
As the top membranes is brought into contact with the bottom membrane, the
mechanical coupling between the square opening on the top layer and the pyramid on the bottom
layer forces the top layer into alignment and prevents further downward movement.
65
In the actual supercapacitor devices fabricated, square pyramids with base dimensions
of 48 pm x 48 pm will used in conjunction with square openings that have a side length
of 15 pm. Based on these dimensions, a membrane separation distance of approximately
14 pm is obtained.
Increased surface area
2.4.2
In addition to improved spacing and alignment, the pyramid shapes can be used to
increase the total area of the supercapacitor's electrode region. For example, if a
400 pm
x
400 pm electrode region is completely covered with 0.5 pm
x
0.5 pm base
pyramids formed using the process mentioned in Section 2.4.1, the total surface area of
the region would increase from 1.6 x 10- 7 m 2 to 6.3 x 10-7 m 2 for an increase of 300%.
To reduce the complexity of the fabrication process, actual origami supercapacitor
devices will be patterned with 3 pm
x
3 pm base pyramids that are spaced 3 pm apart
from one another. Attempting to create much smaller features in the MTL would require
the use of more expensive masks and a more intricate fabrication process. Although the
increase in surface area would be only around 3% with the dimensions used, these
pyramids would nevertheless serve to demonstrate the feasibility of such a technique.
66
Chapter 3
Fabrication
As discussed in previous chapters, the Nanostructured OrigamiTM 3D Fabrication and
Assembly Process is used to create a type of an electrochemical energy storage device
called a supercapacitor. This chapter describes the required fabrication process for such a
device using the materials discussed in Section 2.2.
3.1
Fabrication process
The basic process flow for the origami fabrication method is shown in Figure 3-1. A
more detailed listing of all the processing steps performed and equipments used is given
in Appendix B. A detailed layout of the various origami designs is given in Appendix C.
Starting with a 150 mm (6-inch), (100) silicon wafer, a 2000 A thick layer of silicon
nitride is deposited using low-pressure chemical vapor deposition (LPCVD) process and
subsequently patterned to make a mask for KOH etching. LPCVD silicon nitride was
chosen as a mask because it was found to be highly resilient against KOH. KOH etches
the {100} and {1 1} planes of silicon significantly faster than the {111} plane and can
thus be used to anisotropically etch silicon. This anisotropic etch results in a pyramidal
pit in the silicon as the slow-etching {111 } plane makes a 54.74' angle with respect to the
surface of the wafer. The illustrations in Figures 3-la and 3-2a show the inverted pyramid
shapes that will be etched into the silicon using this method. The larger pyramids are used
67
to achieve higher alignment precision (Section 4.2), and the smaller pyramids are used to
increase the surface area of the electrode region (Section 4.1.3).
Following the KOH etch step, the metal layer is deposited using an e-beam evaporator
(Figures 3-lb and 3-2b). Because gold doesn't adhere very well to either silicon or SU-8,
a 30 nm film of chromium is deposited below and above the 1.5 pm thick gold layer.
Chromium adheres readily to silicon and gold and is commonly used as an adhesion layer
between the two materials. The excellent adhesion properties of chromium are due to its
reactivity; it readily forms a thin and stable oxide coat that prevents further oxidation, the
main cause of poor adhesion [96]. Although the adhesion property of SU-8 to chromium
was not clearly known, it was hypothesized that it would be better than depositing SU-8
directly on top of gold since gold doesn't adhere very well to many different materials. It
was actually found out experimentally that the 2000 series SU-8 manufactured by MicroChem that was used throughout the experiment adheres quite well to many different
materials as stated in product literature [86]. Nevertheless, the top chromium layer is left
on some devices to demonstrate the strain-induced curling of hinges as mentioned in
Section 2.3.3. The gold and chromium layers are patterned through wet etching. Transene
Gold Etchant TFA is used to etch the gold layer, and Cynatec CR-7 Chromium Photomask Etchant is used for the chromium layer. Because there are three different layers of
chromium, gold, and chromium, three separate etching steps need to be performed using
a single photoresist etch mask. A lift-off process was initially planned for the metal
patterning step, but it was abandoned because lifting-off such a thick metal layer turned
out to be impossible using the type of negative photoresist available in the MTL.
Illustrations in Figures 3- Ic and 3-2c show the coating and patterning step of the SU-8
layer. SU-8 2025 from the MicroChem Corporation is applied at a coating spin speed of
3000 RPM to obtain a 25 im thick layer of SU-8. The SU-8 processing step actually
turned out to be one of the most sensitive and difficult since we needed to minimize
crosslinking. More detailed SU-8 processing steps are outlined in Section 3.2.1.
Figures 3-ld and 3-2d show the final release step for the origami devices. Xenon
difluoride (XeF 2) isotropic etching is a room-temperature process that can be used to
completely etch away the silicon lying below the devices to be released. The exothermic
etch reaction between XeF 2 and Si is given by
68
2 XeF 2 + Si-2Xe+ SiF 4 [97].
Reaction 3.1
Using XeF 2 etching as a release step can be very powerful because it usually does not
require an additional sacrificial layer, exhibits extreme etch selectivity over most other
materials, and the use vapor-phase etching completely eliminates stiction, which can be
very problematic in a wet release step.
(a)
(b)
(c)
(d)
Silicon
Gold
SU-8
Figure 3-1: Side profile illustration of the process flow for the origami fabrication of nanostructured electrochemical capacitors. (a) KOH is used to etch pyramidal cavities into the silicon
substrate. (b) Metal layer for the hinges and various wiring is deposited via e-beam evaporation
and patterned with wet etching. (c) SU-8 layer is spun on and patterned to serve as the structural
material. (d) XeF 2 gas is used to isotropically etch away the underlying silicon and release
the device.
69
(a)
(b)
(c)
(d)
Silicon
0 Gold
M SU-8
Figure 3-2: Top view of the process flow shown in Figure 3-1.
The final step after releasing requires folding of the membranes and deposition of the
carbon electrode material as shown in Figure 3-3. For most devices, folding is done
manually using the probe station setup shown in Figure 3-4. Because Lorentz force based
actuation has been demonstrated in the past [26], we felt that it was not necessary to use it
in assembling every single device. In order to make electrochemical cell, two folds must
be made because the gold electrode surface that needs to be painted with carbon is on the
bottom side of the released SU-8 flaps. Although initially having the electrode area on
top of the SU-8 would eliminate the need for making two folds, the current configuration
simplifies the fabrication process and allows the electrode surface to have non-planar
architecture (e.g. 2D array of tiny pyramids). The first fold exposes the gold electrode
70
surface for painting, and the second fold brings together the two painted electrode surfaces to form an active electrochemical cell. Depositing the carbon electrode material is
also done manually using a fine wire.
Although various latching schemes for permanently fixing the folded membranes is
discussed in Section 2.1.4 and implemented in some of the devices that will be mentioned
in Section 4.4, majority of the folded supercapacitor devices were permanently held in
place with a small drop of manually deposited, highly aqueous adhesive, such as liquid
super glue. A small drop of the liquid adhesive less than 100 pm in diameter is applied
between the folded layers using a fine metal wire. As will be discussed later in Section
4.1.3, the devices will actually not function properly without this adhesion step.
(a)
(b)
One Active Electrochemical Cell
(C)
E Silicon
0 Gold
U SU-8
0 Carbon Paint
Figure 3-3: Folding and painting of a supercapacitor following release. (a) The released device
after XeF2 etching. (b) First fold reveals the gold electrode surface, which can then be painted
with a carbon paint mixture. (c) Second fold brings together the painted surfaces to form one
active electrochemical cell.
71
Figure 3-4: Probe station setup used for manual assembly of the origami supercapacitors.
3.2 Processing details
Some of the fabrication steps outlined in the previous section required extra attention
during processing. In addition, creating a test-ready device required further postfabrication processing outside the cleanroom. This section will provide a more detailed
look at some of the processing steps.
3.2.1
SU-8 processing
As mentioned before, SU-8 processing turned out to be one of the most difficult elements
of the origami fabrication process because the level of SU-8 crosslinking must be just
right in order for the devices to function properly. If there is too much crosslinking, the
membranes will shrink excessively in the lateral direction and tear apart the gold hinges.
If there is too little crosslinking, the SU-8 will be too weak and will wash away during
72
developing and rinsing. A nonuniform crosslinking density across the thickness of the
SU-8 will cause it to warp.
As seen in Figure 3-5, lateral shrinkage of the SU-8 flaps during release will easily
tear apart the gold hinge. Therefore, every effort was made to reduce the amount of
shrinkage by minimizing the level of cross-linking in the SU-8. The drawing in Figure 36 shows some of the dimensions of a two-flap device that can change as a result of SU-8
shrinkage. As the surrounding SU-8 shrinks,
iT
and
WT
will increase, and
iF will
decrease
as the square SU-8 piece shrinks. The two stars in the figure indicate the last points of
release for the device; all shrinkage prior to complete release will occur with respect to
these two anchor points. As a result, the increase in iT and WT and the decrease in
IF will
cause the gold hinges to be stretched considerably. Table 3.1 shows typical SU-8 shrinkage in a two-flap device that was fabricated with moderate level of cross-linking. The
shrinkage values shown can cause the hinges to stretch to almost 110% of their initial
lengths and therefore must be reduced. There are basically 8 steps in a conventional SU-8
process: substrate pretreat, coat, soft bake, expose, post exposure bake (PEB), develop,
rinse and dry, and hard bake. Of these 8 steps, expose, PEB, and hard bake steps are
responsible for the degree of cross-linking and need to be carefully adjusted to minimize
shrinkage. Because the appropriate SU-8 recipe depends on so many different factors
such as substrate material, desired thickness, type of heating system, temperature and
humidity of the processing environment, and exposure system, a new set of fabrication
parameters must be created for each desired application. For our devices, a suitable recipe
was generated by going through an extensive test matrix that included all possible
combinations of exposure time, PEB temperature/time, and hard bake temperature/time in
the relatively controlled environment of the MTL. Hard bake is generally avoided in
many MEMS applications because it further increases cross-linking and can lead to high
internal stresses in the final structure. Furthermore, SU-8 is very robust even without this
extra step and usually does not require it for improved mechanical properties unlike other
photoresist-type epoxies. In our case, it was included in the test matrix because it proved
to be effective in removing tiny surface cracks that can appear on the SU-8 structures by
redistributing some of the internal stresses.
73
Figure 3-5: SEM image of a gold hinge that has been stretched and broken during the XeF 2
release process.
A1
*
1T~
'WT
Figure 3-6: Dimensions of a two-flap device that can change as a result of SU-8 shrinkage. The
two stars indicate last points of release for the SU-8 flaps. Shrinkage will occur with respect to
these two anchor points.
74
Table 3.1: Approximate dimensions of the two-flap SU-8 device before and after the release step.
Dimension
Before Release
After Release
% Change
IT
1656 pm
1670 pm
+0.85%
WT
850 pm
861 pm
+1.3%
LF
750 pm
743 pm
-0.93%
After going through the extensive test matrix, a suitable recipe for a 25 pm thick layer
of SU-8 2025 was developed. Wherever possible, processes inducing cross-linking were
minimized, and sudden temperature changes were avoided. The detailed process is listed
in Table 3.2. A final hard bake step is omitted because it was discovered that small
surface cracks, which can be removed through hard bake, did not adversely affect the
final, released SU-8 structure.
Table 3.2: Fabrication process for 25 pm thick layer of SU-8 2025.
Step #
Process
1
Dehydrate for 1 hour in oven at 200 0C
2
Dispense SU-8 by pouring directly from bottle
3
Spread for 30 seconds at 500 RPM
4
Coat for 30 seconds at 3000 RPM
5
Ramp from room temperature to 70 0C on hotplate and hold for 1 minute
6
Heat for 5 minutes on hotplate set at 100 "C
7
Heat for 15 minutes in oven set at 95 'C
8
Expose for 16 seconds (two 8 second exposures with 60 second rest in between)
9
Heat for 1 minute on hotplate set at 65 *C and immediately turn off hotplate
10
Heat for 1.5 minute on hotplate set at 95 0C
11
Slowly cool down on hotplate from Step #9 for approximately 45 minutes
continued on next page
75
Table 3.2: continued
12
Develop in PM Acetate for approximately 4 minutes (Use ultrasonic bath if fine
features need to be developed)
13
Rinse with fresh PM Acetate
14
Dry by spinning for 30 seconds at 3000 RPM
For second-generation supercapacitor devices, which will be mentioned in Section 4.5,
a 15 pm thick SU-8 layer is needed. Unfortunately, reducing the SU-8 thickness from
25 pm to 15 um wasn't as simple as increasing the coating spin speed. A new type of
SU-8 was required along with a whole new test matrix and fabrication process. The
detailed processing steps for creating a 15 jim thick membrane layer using SU-8 2015 is
outlined in Table 3.3.
Table 3.3: Fabrication process for 15 ptm thick layer of SU-8 2015.
Step #
Process
I
Dehydrate for I hour in oven at 200 "C
2
Dispense SU-8 by pouring directly from bottle
3
Spread for 30 seconds by ramping from 0 to 500 RPM
4
Coat for 30 seconds at 3000 RPM
5
Heat for 2 minutes on hotplate set at 65 0 C
6
Heat for 2 minutes on hotplate set at 95 C
7
Heat for 15 minutes in oven set at 95 'C
8
Expose for 11.5 seconds
9
Heat for t minute on hotplate set at 65 C
10
Heat for 1 minute on hotplate set at 95 0C
11
Turn off hotplate and cool on hotplate for approximately 45 minutes
12
Develop in PM Acetate for approximately 4 minutes with very mild agitation
13
Rinse with fresh PM Acetate
continued on next page
76
Table 3.3: continued
14
Dry by spinning for 30 seconds at 3000 RPM
15
Ramp from room temperature to 180 *C on hotplate and hold for 10 minutes
16
Cool down on hotplate for approximately 1 hour
The main difference between the two recipes, besides the obvious reduced exposure
time for the thinner layer, is that a final hard bake step is required for the thinner 15 jim
thick SU-8 layer. The 2000 series of SU-8 used throughout the experiment is optimized
for near UV exposure (350 nm - 400 nm) and has high actinic absorption below 350 nm
[86]. Because the UV exposure system used in the MTL was not fitted with any sort of a
filter, undesirable shorter wavelengths inevitably reach the SU-8 layer and become
readily absorbed in the top surface. It is believed that this causes more Lewis acids to be
generated near the surface and thus leads to a higher cross-linking density in that region.
During the development process, the more highly cross-linked top surface naturally
experiences greater internal stress. The higher internal stress in turn leads to surface
cracking and membrane warping. In the thicker SU-8 membrane, only the surface cracks
appeared, and warping did not occur. It is hypothesized that the lack of warping is due to
the large bulk of SU-8 beneath the thin, overexposed surface that prevents the stressed
top layer from bending the entire structure. Figure 3-7 shows SEM images of an unreleased, 15 pm thick, one-flap device fabricated without (Figure 3-7a) and with (Figure 37b) the hard bake step.
77
(a)
(b)
Figure 3-7: SEM images of unreleased, 15 pim thick, one-flap device fabricated (a) without and
(b) with the hard bake step. The hard bake step relieves some of the stress in the top surface
effectively removing surface cracks and reducing warping.
3.2.2
Release step
As mentioned previously, one of the main advantages of XeF 2 etching is its extremely
high etch selectivity towards silicon. In fact, XACTIX, one of the leading manufacturers
of commercial XeF 2 etching systems, claims a "nearly infinite selectivity" to silicon over
almost all other semiconductor processing materials [98]. An etch selectivity greater than
1000:1 to silicon with respect to other materials such as silicon dioxide, photoresist, and
most metals is commonly accepted [99]. However, Figure 3-8 shows that a gold surface
on the supercapacitor device is partially attacked by XeF2 etching. Furthermore, Figure 39 shows that XeF 2 etching can greatly contribute to hinge failure.
(b)
(a)
Figure 3-8: Microscope images of a gold surface (a) before and (b) after approximately 30
minutes in the XeF2 etch chamber.
78
(a)
(b)
Figure 3-9: SEM images of a gold hinge after XeF 2 etching. (a) The hinge is stretched beyond
failure and also severely etched. (b) The hinge is almost completely etched away.
Although XeF 2 does not appear to etch gold at a significant rate, XeF 2 's effect on gold
clearly cannot be ignored, especially since such a large amount of silicon, up to 400 Pm
laterally in some devices, needs to be etched away. Assuming a relatively high etch
selectivity of 1000:1, this would still result in a 0.4 pm reduction in gold layer thickness.
While hinges shown in Figure 3-9 represent worst case scenarios and over half of the
hinges actually survived the release process, all hinges showed some signs of XeF 2 attack.
Consequently, the thickness of the gold layer was increased further from 1.5 pm to 2 pm.
Interestingly, the amount of gold damage caused by XeF 2 etching varied considerably
from batch to batch. While the exact mechanism of gold etching is not known, it is
conjectured that one or both of the byproducts of the reaction between silicon and XeF2
(Reaction 3.1) may increase the etch rate of gold. This requires a brief explanation of the
XeF 2 etch system used in the MTL.
The SE Tech ES-2000XM XeF2 etcher consists of three chambers: source chamber,
expansion chamber, and etch chamber. The source chamber houses a solid XeF 2 source,
the expansion chamber provides a constant pressure supply of XeF 2 vapor, and the etch
chamber contains the actual devices to be etched.
For each etch cycle, the XeF 2 gas
moves from the expansion chamber to the etch chamber and etches the devices for
typically 30 to 120 seconds. During this time, SiF 4 gas forms inside the etch chamber and
79
increases the chamber pressure. A significant decrease in the rate of SiF 4 gas production
(monitored through a pressure sensor located inside the etch chamber) indicates that most
of the XeF 2 gas in the chamber has been used up. At this point, the etch chamber is
evacuated, and the cycle is repeated. Anywhere from 20 to 100 such cycles are needed to
fully release a supercapacitor device. In release batches that exhibited a significant
amount of gold damage, the etch length, or hold time, during each cycle had been determined by the approximate time it took for most of the XeF 2 gas in the chamber to be
consumed through the reaction. For a 120 second hold time in the etch chamber, a dozen
1 cm 2 dies need approximately 50 cycles to become released, requiring a total hold time
of 100 minutes. On the other hand, batches that exhibited minimal gold damage were
etched with very short hold times, meaning that the etch chamber was evacuated with
much unconsumed XeF2 still remaining inside. For a 30 second hold time, a dozen I cm2
dies need approximately 60 cycles to become released, for a total hold time of 30 minutes.
Clearly, most of the silicon becomes etched during the early part of the holding step.
Although the latter method wastes a lot of unused XeF2 gas, minimizing the total hold
time proved to be effective in decreasing gold damage during release. Figure 3-10 shows
a 2 im thick hinge on a minimally cross-linked SU-8 device released using the procedure
mentioned above.
Figure 3-10: SEM image of an intact 2 jim thick hinge after XeF2 release.
80
3.2.3
Wafer dicing
Before the devices are released in the XeF 2 release step, the wafer needs to be cut into
approximate 1 cm x I cm dies. At first, the wafer, just prior to the release step, was cut
into individual dies using the die saw at the MTL according to standard procedure.
However, the die saw process left a large amount of debris on the SU-8 surface rendering
the devices useless. Subsequently, the wafer was coated with a 10 pm thick photoresist
layer in order to protect the surface during the cutting process. A protective photoresist
layer is commonly used for this purpose and easily removed afterwards by soaking in
acetone. Unfortunately, soaking the devices in acetone had the tendency of completely
lifting off the SU-8 layer from the silicon.
Other batch-fabrication methods of wafer cutting are available commercially, but for
the purposes of this work, the wafers were manually cleaved using a diamond scribe and
a pair of commercial glass cutting pliers. The diamond scribe is first used to score the
wafer, and the glass pliers are used to cleanly break the wafer along the scored lines. This
method proved be very quick and highly effective in generating 1 cm x 1 cm dies from
the 150 mm wafer.
3.2.4
Carbon electrode
The carbon paint mixture for the electrode is made by mixing 99wt% of Super P carbon
black made by TIMCAL with lwt% of polyvinylidene fluoride (PVDF) binder in the
solvent N-Methyl-2-pyrrolidone (NMP). The carbon black itself has a very large surface
area of approximately 62 m2/g [100] due to its porous structure and nano-sized particles
as shown in Figure 3-11.
A single drop of carbon paint is deposited manually onto the gold electrode area using
a very fine metal wire. Using this method, a drop of carbon paint approximately 400 pm
in diameter can be consistently deposited. Once all the solvent has evaporated away from
the carbon paint mixture, a thin carbon film is left on the gold surface. From profilometry
data as well as visual inspection, the thickness of the remaining carbon film is in the
order of a few microns. A more consistent film thickness will require a more precise
81
method of deposition using a very small volume pipette. Figure 3-12 shows the carbon
film deposited on a gold surface.
Figure 3-11: SEM image of the carbon paint mixture (99wt% Super P and Iwt% PVDF) showing
its porous structure and nano-sized particles.
Figure 3-12: Microscope image of the carbon film left on a gold surface after all the solvent is
evaporated away.
82
3.2.5
Packaging
In order to test the completed supercapacitor devices using conventional electrochemical
analysis tools, the final die must be appropriately packaged. An additional packaging
challenge is posed by the requirement that the folded device must be immersed in an
electrolyte solution throughout the electrochemical testing process.
Once the supercapacitor device on the die is fully painted and folded, the die is
secured via epoxy to a side braze-type ceramic chip holder, also known as a dual-in-line
package (DIP). These ceramic packages are used commonly in IC packaging applications
and allow easy wire bonding from the chip to the holder. A gold wire bonder is used to
electrically connect the two bond pads on the die to the DIP. In the final packaging step, a
1 cm tall silicone well is formed around the entire device to create a reservoir for the
electrolyte. A RTV 108 silicone from GE Silicones was used to create the reservoir. All
the packaging materials, such as the ceramic holder, epoxy, gold wire, and silicone, were
tested to make sure that they would stand up to hydrochloric acid (HCl), potassium
hydroxide (KOH), and sulfuric acid (H2SO4), three types of electrolyte solutions that are
commonly used in supercapacitor testing. Figure 3-13 shows the completed supercapacitor package prior to the addition of electrolyte.
Figure 3-13: Image of the completed supercapacitor package, ready for testing.
83
84
Chapter 4
Fabrication Results and Testing
Following the fabrication steps outlined in Chapter 3, an assortment of different origami
devices were successfully fabricated. This chapter shows many of these devices, focusing
mainly on the origami supercapacitor. In addition, the effectiveness of the incorporated
alignment, actuation, and latching mechanisms are discussed.
4.1
4.1.1
Released devices
Two-flap supercapacitor devices
Most of the fabricated origami devices were of the two-flap supercapacitor variety. These
are the devices outlined previously in Section 3.1 where two SU-8 segments are released
from the substrate and folded twice in order to create one active electrochemical cell.
Figure 4-1 shows a pre-released, two-flap supercapacitor with various current loops for
appropriate Lorentz force actuation of the segments. These self-actuating devices, however, were fabricated mainly to demonstrate the possibility of integrating Lorentz force
actuation mechanisms and were not used to create the actual supercapacitors used in final
electrochemical analysis. Because manual carbon paint and epoxy deposition is already
required to create the final device, manual folding of the membranes is actually much
faster and effective in our case.
85
Figure 4-1: Microscope image of the two-flap supercapacitor device with two separate current
loops for Lorentz force folding of the two segments.
Figure 4-2 shows the two-flap supercapacitor device just prior to complete release.
These devices are used to create the many supercapacitor samples required for electro-
chemical testing and need to be folded manually. The shiny region underneath the two
flaps is the silicon trench created by XeF 2 vapor seeping through openings in the SU-8.
Figure 4-2: Microscope image of the two-flap supercapacitor device without current loops for
Lorentz force actuation.
86
Figure 4-3 shows a two-flap supercapacitor device after the XeF 2 release step. Interestingly, many devices popped up out of the substrate upon release without any manual
manipulation. The cause of this effect will be discussed further in Section 4.3.2. Figure 44 shows the same device after the first fold required to expose the gold electrode region.
The electrode areas have been painted with the carbon mixture.
(a)
(b)
Figure 4-3: Microscope images of the two-flap supercapacitor device upon complete release
viewed from the (a) top and from the (b) side. The carbon paint has not yet been applied.
Figure 4-4: Microscope image of the two-flap supercapacitor device after the initial fold and
application of carbon paint.
87
Figure 4-5 shows the completed supercapacitor device after full assembly. The painted
membranes have been folded once more and secured with liquid epoxy.
(a)
(b)
Figure 4-5: Microscope images of the two-flap supercapacitor device after complete assembly
viewed from an (a) angle and from the (b) top.
Unfortunately, the completed two-flap supercapacitor device shown in Figure 4-5, and
other devices sharing the same design, could not be tested for electrochemical performance because the two carbon electrode surfaces touched upon folding, thus creating a
short circuit. The spacing between folded SU-8 layers, and thus between opposing
electrodes, was designed to be around 14 pm using the alignment pyramids mentioned in
Section 2.4.2. However, the final thickness of deposited carbon was usually much thicker
than the initially expected 1 - 3 pm. In some cases, the manually applied carbon layer was
over 10 pim thick! Large lumps and peaks were also found occasionally in the deposited
carbon layer. The recipe for carbon paint given in Section 3.2.4 is actually an improvement over the original recipe that was causing these problems (99wt% Super P and lwt%
PVDF as opposed to 90wt% Super P and 1 Owt% PVDF) and gives a much thinner and
uniform carbon layer. The new and improved carbon paint mixture, however, is still not
enough to resolve the problem. The shorting is exacerbated by the fact that the spacing
pyramids shrink (due to SU-8 shrinkage), and the SU-8 membrane bows outwards ever so
slightly. Both of these occurrences contribute to reduced electrode-to-electrode separa-
88
tion distance. Finally, the folded hinges fail to provide any mechanical support to the
membranes. As mentioned in Section 2.4.2, the spacing pyramids occupy only one side
of the flap, and the hinges were expected to keep the other side of the flap separated.
However, the hinges become very weak after folding and do not prevent the top flap from
sagging on that side. Figure 4-6 shows a side view of a folded, two-flap device (the
bottom half of the image is a reflection of the top half). It can be seen that layer-to-layer
separation is clearly much larger on the spacing pyramid side of the device.
Figure 4-6: Microscope image showing the side view of a folded, two-flap supercapacitor device.
The bottom half is a reflection of the top half. It can be seen that membrane separation distance is
much greater on the pyramid side of the device.
4.1.2
One-flap supercapacitor devices
Fortunately, an alternate supercapacitor design was included in the same mask layout as
the two-flap supercapacitor devices. The one-flap supercapacitor, as its name suggests,
has only one folding flap but works in much the same way as its two-flap counterpart.
The fabrication process is exactly the same as before, and these devices were actually
fabricated in parallel with the two-flap devices on the same wafer. The only change to the
process occurs during the painting and folding steps. Figure 4-7 shows the fabrication,
painting, and folding processes for the one-flap supercapacitor device.
89
(a)
(b)
(c)
(d)
(e)
(f)
Silicon
N Gold
U
SU-8
U
Carbon Paint
Figure 4-7: Side profile illustration of the process flow for the fabrication, painting, and folding
of an one-flap supercapacitor device. (a) KOH is used to etch small pyramid shapes into the
silicon substrate. (b) Metal layer for the hinges and various wiring is deposited via e-beam
evaporation and patterned with wet etching. (c) SU-8 layer is spun on and patterned to serve as
the structural material. (d) XeF2 gas is used to isotropically etch away the underlying silicon and
release the single flap. (e) Carbon paint is manually deposited on the gold electrode surface. (f)
The single released flap is folded.
90
The one-flap design offers several advantages over the two-flap design. First of all, the
gold electrode area is on the top surface and thus exposed prior the XeF 2 release step.
Conversely, the two-flap devices require one initial fold to expose the electrode area that
is on the underside of the SU-8 membrane. In addition to greatly simplifying the manual
painting and folding process, this new configuration will allow the use of soft lithography-type techniques for depositing the carbon layer during the fabrication process. In the
two-flap design, batch-fabrication-type carbon deposition cannot occur before the release
step since the electrode region is not accessible without the first fold and cannot occur
after the release step since the devices are too fragile to undergo further cleanroom
processing once the flaps are released. Second, the electrode region in the one-flap design
is recessed below the surface and is effectively surrounded by a 25 pm thick wall of SU-8.
This guarantees a electrode-to-electrode separation distance of at least twice the SU-8
membrane thickness and prevents shorting. Whereas the electrode separation distance is
controlled by the size of the spacing pyramids and square openings in the two-flap design,
thickness of the SU-8 layer controls the separation distance in one-flap devices. Also, the
SU-8 well around the electrode region effectively confines the carbon paint mixture upon
deposition and prevents the mixture from flowing out to other parts of the device (Figure
4-8). Figure 4-9 shows the one-flap supercapacitor device painted and ready for folding.
(a)
(b)
Figure 4-8: Microscope images of the carbon painted electrode area in (a) two-flap and (b) oneflap supercapacitor devices. The SU-8 wall helps confine the carbon paint within the gold area in
the one-flap device while some of the carbon in the two-flap device is touching the adjacent wire.
91
Figure 4-9: Microscope image of the one-flap supercapacitor device after carbon paint deposition.
The released flap on the bottom needs to be folded over to complete the assembly.
4.1.3 Elastic spring-back
In Section 2.2.2, it was mentioned that the gold hinges are susceptible to elastic springback. Also, it was mentioned in Section 3.1 that an adhesive is required to keep the
membrane in the folded position. The presence of elastic spring-back suggests that the
gold hinges are not fully plastically deformed upon 1800 folding.
Elastic spring-back is of great importance in many industrial applications such as
metal-forming and pipe-bending. The spring-back angle 6, can be easily derived using
standard beam bending analysis [101] and can be calculated by using the equation
3u0, = ORO y
(4.1)
(Eh)
where Qo is the desired bend angle, Ro is the bending radius, a, is the yield stress of the
hinge material, E is its Young's modulus, and h is the hinge thickness. From Equation
(4.1),
it can be seen that a smaller bending radius, thicker hinge layer, and a material
with a low yield strain (where yield strain c. = u, / E) will all contribute to a reduced
92
spring-back angle. Given the parameters of the hinge given in Table 4.1, an approximate
spring-back angle of Os
theoretical=
230 can be expected.
Table 4.1: Parameters of the gold hinge.
Parameter
Value
00
1800
Ro
25 pm
1-y
E
206 MPa [50]
h
1.5 pm
79.4 GPa [50]
Microscope images shown in Figure 4-10 show overhead views of a device without
and with elastic spring-back. In the first device, no spring-back is exhibited because all
the gold hinges are broken or just barely hanging on. In the second device, comparing the
apparent length and width of the square flap obtained from the image indicates that an
approximate elastic spring-back angle of Os ,acttal = 250 is demonstrated. The calculated
and experimentally obtained values are very similar, and it is clearly seen that some type
of a latching mechanism is required to hold the folded flaps in their final positions.
(b)
(a)
Figure 4-10: Microscope images of a flap folded over 180'. (a) no elastic-spring back is
demonstrated due to broken or almost-broken hinges. (b) Elastic spring-back is shown.
93
4.2
Pyramid structures
As mentioned in Section 2.4, two types of pyramid structures are incorporated into the
supercapacitor design. Figure 4-11, shows a single square flap of the origami supercapacitor. The two spacing and alignment pyramids are shown on the left, and the square
region in the center is covered with 625 smaller pyramids.
Figure 4-11: SEM image of a single square flap on the origami supercapacitor.
4.2.1
Increased surface area
As mentioned in Section 2.4.2, the gold electrode region is covered with a 2D array of
3 pim x 3 im pyramids that are spaced 3 pm apart. Figure 4-12 shows that these pyramids
were actually severely overetched, resulting in a pyramid base of approximately
94
5 im
x
5 im and a separation distance of approximately 1 jim. The overetch occurred
because the significantly larger spacing and alignment pyramids required a much longer
etch time and forced the devices to be left in KOH long after the {111 } planes had
already intersected in the smaller pyramids. The increased size of these pyramids actually
caused the surface area of the gold electrode region to increase by 7% as opposed to the
initial calculation of 3%
Figure 4-12: SEM image of the supercapacitor's electrode region before the deposition of carbon
paint. The array of pyramids help increase the surface area.
4.2.2
Spacing and alignment
The fabricated spacing and alignment pyramids are shown in Figure 4-13. As mentioned
before, these pyramids were ineffective in maintaining a separation distance between the
two opposing electrodes in the two-flap supercapacitor device because the spacer pyramids were placed on only the unhinged side of the SU-8 membrane; the gold hinges on
the other side failed to provide any mechanical support and caused the top flap to sag in
that region. In future devices, these pyramids should be placed on all four sides to maintain a uniform separation distance across the entire membrane.
95
Figure 4-13: SEM image of the spacing and alignment pyramids.
Figure 4-14 shows a top-down view of the alignment pyramid and the square opening.
The SEM image shows that the tip of the pyramid is at almost the exact center of the
square opening. In fact, measurements taken in the SEM indicate that the tip shown in the
figure is within I pm of the true center point in both x and y directions. Figure 4-15
shows the same view for a different device in which the pyramid tip is slightly more offcenter. Even so, the pyramid tip is still within 2 pm of the actual center point of the
square opening in both x and y directions.
Figure 4-14: Top-down SEM image of square opening fitted over an alignment pyramid. Alignment error is around I im.
96
Figure 4-15: Top-down SEM image of square opening fitted over an alignment pyramid. Alignment error is around 2 pm.
Alignment precision between entire flaps is a bit more difficult to determine. As
Figure 4-16 shows, the edges of the SU-8 membrane are not perfectly straight, and the
sidewalls are slightly angled. It's hard to tell whether the apparent misalignment between
the top and bottom flaps, which should be identical in lateral dimensions, is due to the
inadequacy of the alignment system or nonuniformity in the SU-8 membrane. In addition,
a slight curvature in the membrane makes this task even more difficult. Separate alignment markers should be included in future devices to allow a more precise determination
of alignment error between folded layers.
Figure 4-16: SEM image of a folded, two-flap supercapacitor device.
97
Finally, future devices should utilize the principle of elastic averaging to increase
alignment precision between membranes. The principle of elastic averaging states that the
number of contact points between two surfaces should be maximized and spread out over
a broad region in order to more accurately locate the two surfaces and support a larger
load [74]. While the origami devices tested included only two such contact points, future
devices should have as many alignment mechanisms as possible for greater alignment
precision between folded layers.
4.3 Actuation
Although all of the supercapacitor devices intended for electrochemical testing were
assembled manually, two types of actuation methods, magnetic and stress-induced, were
explored.
4.3.1
Magnetic actuation
The magnetic, or Lorentz force-based, actuation method had been used previously [26] to
demonstrate a 1800 fold. The drawing in Figure 4-17 illustrates how Lorentz force is used
to fold the flaps in the origami devices. First, a magnetic field parallel to the substrate and
in the direction shown in the figure is attained using an external horseshoe magnet.
Subsequently, appropriate current is applied to the device thus causing an upward force to
act on the membrane to be folded. Theoretically, a maximum folding angle of 900 can be
achieved with this setup, at which point the force becomes parallel to the folding segment
and no longer generates a moment about the hinges. In order to complete the 1800 fold,
the magnetic field must then be rotated downward by 900.
Force
inmJrrent
Figure 4-17: Illustration of the Lorentz force actuation concept.
98
Another method of achieving 1800 folds is to continuously rotate the magnetic field
during the folding process as shown in Figure 4-18. As a result, the rotating moment
about the hinges remains constant throughout the folding process. Since rotation of the
magnetic field is also required with the previous setup, the addition of a continuously
rotating magnetic field does not impose further difficulty. Because this is a parallel
actuation process, it can be used in batch-fabrication applications. The test setup for this
type of folding is shown in Figure 4-19 and Figure 4-20.
Magnetic Field
f Current
FYorce
Magnetic Field k
Current
Force
Figure 4-18: Illustration of the Lorentz force actuation concept with a continuously rotating
magnetic field.
99
Figure 4-19: Test setup for Lorentz force folding with continuous magnetic field rotation. The
device to be tested is suspended in air with a rigid rod to allow the horseshoe magnet to free
rotate around it.
Figure 4-20: Close-image of the suspended device. The horseshoe magnet is not shown.
100
An one-flap actuation device shown in Figure 4-21 a was tested using the setup above.
When the applied current is around 45 mA, the device begins to move slightly. With an
increased current of around 200 mA, the angle of rotation can be controlled precisely by
rotating the horseshoe magnet. However, rotation beyond approximately 100 could not be
achieved, and raising the current further to around 400 mA resulted in the melting of the
SU-8 membrane as shown in Figure 4-2 lb. SU-8 degrades significantly at around 400 'C
[102], and thermal analysis performed in [26] indicates that 400 mA of current flowing
through the gold hinges will heat them up to around this temperature.
(a)
(b)
Figure 4-21: One-flap Lorentz force actuation device (a) before testing and (b) after being melted.
The small range of rotation can be explained by the fact that, in order to increase
fabrication yield of the devices, the gold hinge layer was increased in thickness from
1.5 pm to 2 um. Additionally, a 350 pm wide hinge was added to provide increased
stability during folding. In Section 2.3.2, it was reported that the moment Mmag provided
by Lorentz force should be very similar to the device's required maximum bending
moment of
Mreq.
Taking into account the increase in hinge thickness and width and also
the reduced current required to prevent SU-8 melting, the same analysis indicates that
Mreq is now about ten times greater than Mmag. The use of a stronger magnet with a
magnetic flux density around 1 T could resolve this problem in future devices.
101
Furthermore, the incorporation of an effective latching mechanism in the future could
enable multiple segment folding using the Lorentz force actuation method with continuous magnetic field rotation. Although only the top flap in Figure 4-22 is affected by
Lorentz force, rotating the magnetic field back and forth, as indicated by the blue arrow
in the figure, with constant current flowing through the wire loop results in actuation of
the entire structure. If the flaps could be sequentially latched as shown in the figure,
multi-layer folding can be achieved as the structure folds into an accordion-like geometry.
Although the multi-folding devices were included in the fabrication batch and successfully released, lack of a suitable latching mechanism and the problem with increased
hinge stiffness mentioned above prevented successful, multi-layer folding.
102
'-N
0
B2
B
B
0B
0B
B
I
0B
B
B
Figure 4-22: Illustration of the multi-layer folding process using Lorentz force actuation. If the
magnetic field is rotated back and forth as shown in the figure and the folded flaps latched
sequentially as shown, multi-layered origami devices could be batch-fabricated.
103
4.3.2 Stress-induced actuation
It was mentioned in Section 4.1.1 that many of the released devices had automatically
popped up out of the substrate upon release. At first, it was hypothesized that the chromium adhesion layer on top of the gold hinge layer was responsible (chromium adhesion
layer below the gold layer was assumed to have been etched away with the silicon during
the XeF 2 release step). Even if this were the case, Equation (2.6) indicates that the curling
angle due to the residual stress of chromium would be only around 5'. However, many of
the popped-up flaps, as shown in Figure 4-23, exhibited much larger bending angles, and
some of the devices, as shown in Figure 4-24, exhibited bending angles far greater than
90'. Furthermore, devices that did not have the top chromium adhesion layer also demonstrated this phenomenon. A stress gradient in the gold layer was also suspected, but as
Figure 4-25 shows, the gold layer is relatively stress free and would not contribute to
stress-induced curling of the hinges.
Figure 4-23: Microscope image of a 5-flap device that as popped up out of the substrate upon
release.
104
Figure 4-24: SEM images of an one-flap supercapacitor device that has popped up to an angle of
approximately 1300.
Figure 4-25: SEM image of a 2 pm thick gold layer suspended on a silicon column.
It is conjectured that the popup of the origami flaps upon release is due to complex
edge effects taking place at both ends of the hinge. As illustrated in Figure 4-26, the edge
region of a tensile film attached to a substrate will be bent back from the originally
105
vertical edge to account for the non-zero in-plane force [103]. In our origami device,
shrinkage of the SU-8 layer produces a similar effect at the interface between the gold
and the SU-8. As shown in Figure 4-27, tensile forces in the SU-8 layer will cause the
vertical edge planes of the hinge to become negatively sloped and naturally bend down
anything that is attached to it. The result of finite element analysis (FEA) is shown in
Figure 4-28 and confirms that SU-8 shrinkage will translate into hinge bending. Exact
bending behavior of the hinges based on this principle is hard to predict because of
complex stress distribution in the edge region and very complex shrinkage behavior of
SU-8. However, new stress-induced actuation mechanisms based on this principle may
prove to important in future devices.
F = 04-
-
F * 04-
F =0
(a)
- F=0
(b)
Figure 4-26: Edge region behavior of a tensile film attached to a substrate. (a) No tensile stress is
present in the thin film. (b) Tensile stress in the thin film causes the edge plane to bend.
Figure 4-27: Illustration showing the effect of SU-8 shrinkage on the edge plane of the gold hinge
layer. A stress-free gold bar that is attached to such a plane will be bent downwards.
106
Figure 4-28: Results of FEA showing the upward bending of a stress-free gold layer due to tensile
stress present in the SU-8 layer. The thin layer on the bottom is gold, and the thick layer on top is
SU-8.
4.4
Latching
Because of the elastic spring-back effect, folded flaps tend not to stay in their folded
positions. As a quick remedy for the problem, most of the folded flaps in the supercapacitor devices tested were permanently held in place with a small drop of liquid epoxy. Even
without the spring-back problem, final structures created by the origami method should
be permanently fixed with some type of a latching mechanism.
4.4.1 Mechanical latching
In [80], microrivets and corresponding openings were used to mechanically latch hinged,
polysilicon flaps. A similar design was adopted and incorporated into some supercapacitor devices. Figure 4-29 shows an one-flap supercapacitor device with integrated mechanical latches. Once the bottom flap is folded, the trapezoidal SU-8 piece is designed to
fit inside the rectangular opening on the other side. However, the device shown in the
figure did not work as designed because precise dimensions required for a mechanical
latching systems could not be realized with SU-8, especially one that was patterned using
a transparency mask. In order for the trapezoidal piece to latch properly, its dimensions
107
must be exact; if it is too small, it will just slip right out, and if it is too big, the large
deformation required for it to fit through the rectangular opening will destroy it (which is
what happened in our case). In any case, mechanical latching may not be suitable in
many applications because it requires a relatively large force to initiate the latching
process.
Figure 4-29: Microscope image of an one-flap supercapacitor with an integrated mechanical
latching system. The edges of the devices are outlined in red for clarity.
4.4.2 Photoresist latching
An adhesive-based latching system was also briefly explored on the origami supercapacitors. AZ4620 photoresist is used frequently for its low reflow temperature in MEMS
applications such as surface-tension assembly [59] and fabrication of a microlens array
[104].
As shown in Figure 4-30, a 10 pm thick layer of AZ4620 photoresist is coated and
patterned on top of an one-flap supercapacitor device. Once the device is released, the
single flap is folded over to bring the two photoresist pads into contact with each other.
108
At this point, some of the hinges are manually torn to the point where no more elastic
spring-back is exhibited.
Photoresist Pads
Figure 4-30: An one-flap supercapacitor device with two photoresist pads for adhesive bonding
before the reflow process.
The folded devices are heated on a hotplate for about 15 minutes at approximately
170 *C. After a 15 minutes cooling period, probe tips are used to try and remove the top
flap. Clearly from this experiment, the top flap demonstrates improved adhesion as a
result of the melted photoresist. However, the flap is removed rather easily with the probe
tip. As Figure 4-31 shows, the photoresist on the bottom layer has fully melted while the
photoresist on the top layer has only partially melted. This is likely due to the poor
contact between the two surfaces. Indeed, addition of a slight downward force during the
heating process produced devices that were very difficult to disassemble. Use of a
different adhesive, perhaps one that becomes highly aqueous upon melting, could be used
to improve adhesion and even alignment in future devices.
109
(a)
(b)
Figure 4-31: Photoresist pads after the reflow process. (a) The photoresist pad on the bottom layer
has fully melted. (b) Only a small portion of the photoresist pad on the top layer has melted.
4.5
Second- generation supercapacitor devices
For a thorough analysis of electrochemical performance, a large number of test samples
are needed. For this purpose, a second-generation design of supercapacitors were developed. Most of the electrochemical testing outlined in the next chapter was performed
using the new devices. These devices were designed to increase yield and simplify the
manual assembly process. The new process flow and mask layout can be found in Appendix D and Appendix E, respectively.
One major difference between the first and second-generation devices is the thickness
of the SU-8 layer. In the new devices, a 15 im thick SU-8 layer is used instead of a 25
jim thick layer. The reduced thickness results in decreased electrode separation distance.
Improvements in carbon mixing and deposition techniques ensure that the devices will
not short due to a thick or lumpy carbon paint layer.
Another major difference is the addition of a 350 ,m wide hinge placed between the
two regular hinges. This new hinge offers more mechanical stability during the manual
folding process and also improves the yield by taking some of the stress away from the
other hinges.
110
A large number of etch holes have also been placed strategically throughout the new
devices. Of course, the etch holes are effective in reducing the total etch time required for
release. However, in conjunction with the 350 pm wide hinge mentioned above, these
etch holes help shift the final release point of the SU-8 flap. First-generation devices,
which did not have etch holes or the center hinge, are released from the substrate as XeF 2
seeps in from openings in the SU-8 and etches away the underling silicon isotropically.
Because the XeF 2 vapor comes in equally from all sides, the last point of release from the
silicon substrate is near the exact center of the flap. However, in the new devices, the etch
holes speed up the etching of silicon on the unhinged side, and the center hinge slows
down the etching on the hinged side. As a result, the final release point of the SU-8 flap is
shifted up as shown in Figure 4-32. Because SU-8 shrinkage occurs with respect to these
anchor points, the resulting stretch on the gold hinges is significantly reduced by this shift.
The device in Figure 4-32, for example, is designed to have 50 pm gaps on its hinged and
unhinged sides. Just prior to release, the shrinkage of the rectangular SU-8 piece has
increased the gap on the hinged side to 52 pm and the gap on the unhinged side to 72 pm.
Without the shifted release point, the hinges may not have survived the big stretch.
New Release Point
Old Release Point
Figure 4-32: SEM image of the second-generation supercapacitor with etch holes and a wide
center hinge. The new elements have shifted the etch release point as shown in the figure.
111
Finally, a 2000 A thick silicon nitride isolation layer was added between the silicon
substrate and the metal layer. Although first-generation devices did not have this extra
isolation layer given the relatively high resistivity (~6 f2-cm) of the silicon wafers used, it
was included in the second-generation devices to completely eliminate substrate conductivity that could adversely affect electrochemical testing results. Although silicon nitride
exhibits some resistivity to XeF 2 etching, the release step was virtually unaffected and
further changes to the fabrication were not required as XeF2 quickly etched through the
exposed portions of the relatively thin nitride layer and released the SU-8 flaps as usual.
112
Chapter 5
Electrochemical Testing
This chapter outlines the electrochemical testing methods and results of supercapacitors
created using the Nanostructured OrigamiTM process.
5.1
Experimental process and setup
Before the completed supercapacitor package described in Section 3.2.5 can be tested, the
silicone reservoir must be filled with an electrolyte solution. As mentioned previously,
HC, KOH, and H2 SO 4 are the most common types of electrolytes used in supercapacitor
testing. The use of KOH was quickly abandoned because it is a well-known etchant of
silicon. Supercapacitor devices immersed in different concentrations of HCl and H2 S0
4
were all tested, but the majority of results shown in this chapter were obtained using a
solution of 1 M H2 SO 4 . The different concentrations and types of electrolytic solutions
did not appear to have a significant impact on the final results, and the solution of I M
H2 0S4 was chosen mainly for its wide use in literature.
Three types of electrochemical testing methods, AC impedance spectroscopy, galvanostatic testing, and cyclic voltammetry, are most commonly used to characterize the
performance of supercapacitors. A more detailed explanation of the three testing methods
used can be found in Appendix A.
113
The test setup used for the electrochemical measurements are shown in Figure 5-1. A
Solartron 1260 Impedance/Gain-Phase Analyzer was used to obtain AC impedance
measurements, and a EG&G Princeton Applied Research Potentiostat/Galvanostat Model
263A was used for the galvanostatic and cyclic voltammetry measurements.
Figure 5-1: Experimental setup used for the electrochemical testing of supercapacitors.
114
5.2
Experimental results and discussion
Initially, the completed supercapacitor assembly from Section 3.2.5 was filled with
electrolyte and tested using the three methods mentioned above. The results from those
three tests are presented in this section. It should be pointed out that results shown in this
section are actually from a second round of electrochemical tests. In the first round of
testing, the assembled supercapacitor package consisted of a fabricated die, a ceramic
chip holder, and a silicone reservoir that surrounded the entire system. Subsequent
electrochemical testing indicated that the devices had a very high capacitance given the
electrode area which was only around 350 pm x 350 pm. Interestingly, completed supercapacitor packages in which the gold hinges were completely severed or where the flaps
were completely missing also provided impressive data. It was then conjectured that the
high capacitance value was a result of the relatively large areas of positively and negatively charged metallic surfaces that were contained within the silicone well and the
electrolyte solution. In order to make sure that the electrochemical data taken was only
from the actual device (i.e. folded flaps), a new packaging scheme was utilized in which
the silicon reservoir was only around the folded flaps. Other charged, metallic objects
that used to be in the electrolyte reservoir, such as bond pads, bonding wires, and the
numerous wire bonding sites on the chip holder, should no longer contribute to overall
capacitance observed.
115
Figure 5-2: New supercapacitor assembly used during the second round of testing. The silicon
reservoir surrounds only the folded flaps.
In order to make sure that the carbon electrodes were actually contributing to improved capacitance, identical devices with and without the carbon mixture were both
tested under same test conditions. Figure 5-3 shows the Nyquist plot generated from AC
impedance measurements of devices with high surface area carbon electrodes. Figure 5-4
shows the same plot for a device without carbon (bare gold electrodes). All tests were
conducted over a frequency range of 0.1 Hz to 1 MHz at 10 mV amplitude. Ten points
per decade were collected going from high to lo frequencies. For the device with carbon
electrodes, the first smaller semicircle indicates an approximate capacitance of 4 x 10-9 F
while the second semicircle indicates approximately 5 x 10-7 F. These values were
estimated by fitting a semicircle to the Nyquist plot in Figure 5-3. The equation for the
semicircle is given by [47]
Z =
- (coR 2 C 2 + 1)
-
116
(co2 R 2 C 2 + 1)
(5.1)
where Z is overall impedance, cw is frequency, R is resistance, and C is capacitance. The
presence of two semicircles, as in Figure 5-3, indicates that the system can be modeled by
two parallel RC circuits in series. We conjecture that the first semicircle is a result of the
double layer capacitance that results from the high surface area carbon. Consequently, the
first semicircle is barely noticeable in devices without carbon, and the associated doublelayer capacitance is about an order of magnitude smaller compared to devices with
carbon. It is believed that the second semicircle is a result of pseudocapacitance that
arises from chemisorption of anions for example HS0 4 ions on gold and carbon and also
from intercalation of ions (H+ and HS0 4 ) into high surface area carbon. As a result,
devices without carbon exhibited pseudocapacitance values that are about three orders of
magnitude smaller compared to devices with carbon.
35000
30000
25000
20000
15000
10000
5000
0
0
5000
10000
15000
20000
25000
30000
35000
Zr (a)
Figure 5-3: Nyquist plot generated from the AC impedance measurement of supercapacitors with
carbon electrodes.
117
3.OE+06 ,
2.OE+06
-
1.OE+06
O.OE+00
O.OE+00
1.OE+06
2.OE+06
3.QE+06
4.OE+06
5.QE+06
Z' (0)
Figure 5-4: Nyquist plot generated from the AC impedance measurement of supercapacitors with
carbon electrodes.
In galvanostatic testing, a constant current is applied to the device, and the capacitance
is obtained by using the equation
C-
dVT
dt
(5.2)
where I is the charging current and dV/dt is the change in voltage over time. These tests
simply give a single capacitance value for the entire system, and thus they cannot reveal
the various electrochemical processes that may be present. For devices with carbon
electrodes, a capacitance of approximately 2 x 10-6 F was obtained using this method.
This value is within reasonable range of values obtained previously through impedance
analysis. The plot of voltage versus time obtained from galvanostatic testing is shown in
Figure 5-5.
118
1.2
>1
0.9
0
5
10
15
Time (s)
Figure 5-5: Galvanostatic charge (I = 100 pA) of s supercapacitor with carbon electrodes.
Although the double-layer capacitance obtained from high surface area carbon sur-
faces appears rather low, especially given the much larger pseudocapacitance value, the
obtained value is not unreasonable for the type of carbon used. Theoretically, the double
layer capacitance of carbon black in IM H 2 SO 4 is 8 pFcm-2 [47] and the surface area of
the carbon used in the device is 62 m 2 /g [100]. This leads to an approximate specific
capacitance of 5 F/g. Double-layer capacitance of 4 x 10-9 F obtained from our device
through impedance analysis results in a specific capacitance of approximately 1 F/g. We
believe that the specific capacitance of our device could be improved significantly by
modifying the carbon mixture. For example, using a form of carbon with a surface area of
2000 m2 /g should result in a much higher specific capacitance.
119
120
Chapter 6
Conclusion and Future Work
The main purpose of this thesis was to demonstrate that working electrochemical energy
storage devices could be fabricated using the Nanostructured OrigamiTM process. As a
first demonstration of this concept, origami fabrication of an electrochemical supercapacitor was undertaken. To accomplish this, the previously developed origami process,
which had been based on SOI wafers, was modified to use SU-8 as the structural material.
The use of SU-8 offered many advantages over the old process, such as reduced processing complexity and cost. However, significant polymer shrinkage exhibited by crosslinked SU-8 caused problems during the release step by stretching the elastic hinges
beyond failure. Accordingly, minimizing the cross-linking density of SU-8 and increasing
the cross-sectional area of the gold hinges greatly improved fabrication yield. The
addition of spacing and alignment pyramids helped improve spacing and alignment
precision between folded membranes, and a latching mechanism based on photoresist
reflow was met with limited success. A variation of the Lorentz force actuation method,
in which current through the device is kept constant while the magnetic field is continuously rotated, was also explored and demonstrated.
Electrochemical testing of completed supercapacitor devices showed that a specific
capacitance of around 1 Farad per gram of carbon was obtained. This value was within
one order of magnitude of the calculated theoretical value and within two orders of
magnitude of the specific capacitance exhibited by state-of-the-art, macroscale, handmade electrochemical capacitors. In any case, the obtained value is many orders of
121
magnitude larger than that of a similarly sized electrostatic capacitor and demonstrates
that the Nanostructured OrigamiTM process could be used to create high-performance,
integrated sources of energy for micro- and nanomanufacturing applications.
In future work, the electrochemical performance of the supercapacitors can be improved significantly by using a type of carbon with greater surface area. In addition,
fabrication yield could be improved even further by modifying the design of the hinges.
Figure 6-1 shows the result of finite element analysis on three different types of hinge
design, all with the same length and maximum width. In Figure 6-la, which is the basic
rectangular design used in all previously fabricated devices, stretching of the hinge
results in a uniform stress distribution throughout the structure. The areas shown in red
indicate regions where the yield stress was surpassed, and although a more detailed
failure analysis would be required to be certain, the hinge will most likely fail somewhere
in this stressed region. Figure 6-lb shows a hinge with concave sidewalls. The heavily
stressed area near the center will most likely result in the hinge failing in that region.
Interestingly, the hinge with convex sidewalls shown in Figure 6-1c shows that the highly
stressed regions have been isolated near the ends. Even if the material were to fail in that
region, it is possible that the overall structural integrity of the hinge might still be maintained. If the use of SU-8 as the membrane layer is to continue in the future, mechanisms
for compensating SU-8 shrinkage will have to included in the design. As seen in Figure
6-1, modifying the hinge design could offer one possible solution.
Besides electrochemical energy storage devices, the Nanostructured OrigamiTM
method can be used in many other applications. One example is the 3D photonic crystal
shown in Figure 6-2. Using standard planar nanofabrication techniques, such as e-beam
lithography, an array of 2D photonic crystals can be first fabricated and then folded into a
3D configuration. Membrane spacing and alignment precision will have to be significantly improved, and latching mechanisms will have to be perfected.
122
(a)
(b)
(c)
Figure 6-1: Results of FEA on different hinge designs [105]. (a) rectangular hinge. (b) hinge with
concave sidewalls. (c) hinge with convex sidewalls.
Figure 6-2: The use of the Nanostructured OrigamiTM process in 3D photonic crystal fabrication.
Standard nanofabrication techniques are used to create the array of 2D photonic crystals which
are subsequently folded to create the 3D structure [10].
123
124
Appendix A
Electrochemical Testing Methods
In order to evaluate the performances of electrochemical capacitors fabricated using the
Nanostructured OrigamiTM process, three commonly used electrochemical testing methods were employed. These testing procedures will be briefly explained in this section.
A. 1
Galvanostatic testing
Galvanostatic testing measures the capacitance of a capacitor by applying a constant
current I. In response to this current, charge accumulates on the capacitor at the rate
dQ =1.
(A.1)
dt
Since, the relationship between charge
Q, capacitance
C, and voltage V is given by the
well-known equation
Q = CV,
we can thencombine Equation (A.1) and Equation (A.2) to obtain
125
(A.2)
I=C
C=
dt
I
dVl
(A.3)
(A.4)
dt
Since I is known, capacitance of the device can be obtained by simply measuring its
voltage change over time. This method can be used during the charging cycle as well as
the discharging cycle.
A.2
Cyclic voltammetry
In galvanostatic testing, the device is charged at a constant rate while the voltage response is recorded. Cyclic voltammetry works in the opposite way by sweeping the
voltage at a constant rate between two voltage points and measuring the current response.
Since dV/dt is known, Equation (A.4) can be used once again to obtain capacitance.
In addition to providing a quick capacitance measurement, cyclic voltammetry can
offer some insight into the electrochemical behavior of the capacitor as well. For example,
the presence of oxidation-reduction reactions within the system will show up as sharp
peaks when current is plotted as a function of voltage. Furthermore, the stability of the
system can be tested by continuously cycling between the two voltage points.
A.3
AC impedance spectroscopy
AC impedance spectroscopy is a power measurement tool that enables the capacitance to
be determined as a function of frequency. Using specialized equipment, a small AC
signal is applied to the device, and the changes in magnitude and phase of the device is
recorded over a range of frequencies. This method is especially valuable because it
allows us to construct an equivalent circuit of the complex electrochemical system tested.
This makes it possible to separately evaluate various electrochemical reactions within the
system that may be contributing to the overall capacitance. In the case of supercapacitors,
126
impedance spectroscopy can help us distinguish between pseudocapacitance and doublelayer capacitance over a range of frequencies.
127
128
Appendix B
Process Flow for First-Generation
Supercapacitor Devices
The process flow given in this section outlines the various fabrication steps and equipments that were used for creating the first-generation supercapacitor devices. The included equipment listings refer to those at MIT's Microsystems Technology Laboratories.
Table B. 1: MTL process flow for first-generation supercapacitor devices.
Step
1
Description
Comment
Reciple
Machine
RCA
RCA clean
2000 A PECVD nitride
VTR
- both sides
2
Nitride Dep
3
HMDS
4
Spin Resist
STD resist, 1 pm
coater
5
Prebake
30 min
oven
6 Expose
HMDS
Defines bumps and spacers
Mask 1
EV1
7
Develop
photo-wet-I
8
Spin Rinse Dry
Visual Inspec-
SRD
9
tion
Check closest features
continued on next page
129
Table B. 1: continued.
10
Postbake
30min
oven
11
Nitride Etch
SF6
LAM490B
12
Strip Resist
13
KOH etch
Double Piranha
+ HF dip
14
Remove Nitride
13
asher
Shaillow etch, about
40 microns
KOHhood
DbI Piranha before going
back to TRL/ICL
1 hour
acidhood
nitrEtchHotPhos
SRD
Spin Rinse Dry
Thick Resist Recipe
300 A Cr, 1.5 pm Au,
300 A Cr
HMDS
16
HMDS
Metal Deposition
17
Spin Resist
STD resist, 1 pm
coater
18
Prebake
30 min
15
e-beam-Au
oven
Defines hinges and electrodes
Mask 2
EV1
19
Expose
20
Develop
photo-wet-I
21
SRD-Au
22
Spin Rinse Dry
Visual Inspection
23
Postbake
30min
oven
24
Cr etch
CR-7
acid-hood2
25
Au etch
Transene gold etch
acid-hood2
26
Cr etch
CR-7
acid-hood2
27
Strip Resist
nanostrip
28
Dehydrate
1 hour at 150C
acid-hood2
VarTemp
Oven
29
Spin SU-8
25 microns thick
30
Softbake
31
Expose
32
Post Exp Bake
33
Develop
33
Rinse
Visual Inspection
34
Check closest features
SU8-spinner
hotplate/SU8Oven
Mask 3
Defines SU-8 structural
elements
EVI
Further crosslinks SU-8
hotplate
photo-wet-Au
Use fresh PM acetate
photo-wet-Au
Check closest features
continued on next page
130
Table B.1: continued.
36
Cleave Wafer
37
XeF2 release
use scribe/tweezer
About 60 minutes
XeF2
131
132
Appendix C
Mask Layout for First-Generation
Supercapacitor Devices
ni
E-1
I
LF
Figure C-1: Six-flap, magnetic actuation device.
133
U
FI
Figure C-2: Two-flap supercapacitor device with spacing and alignment pyramids. The red region
in the center indicates the smaller, surface area enhancement pyramids.
U
Figure C-3: One-flap supercapacitor device.
134
Figure C-4: Two-flap supercapacitor device with current loops for Lorentz force actuation.
135
136
Appendix D
Process Flow for SecondGeneration Supercapacitor Devices
The process flow given in this section outlines the various fabrication steps and equipments that were used for creating the second-generation supercapacitor devices. The
included equipment listings refer to those at MIT's Microsystems Technology Laboratories.
Table D. 1: MTL process flow for first-generation supercapacitor devices.
Step Description
1
Recipie
Comment
Machine
RCA clean
RCA
2000 A PECVD nitride
2
Nitride Dep
- both sides
VTR
3
Metal Deposition
300 A Cr, 2 pm Au
e-beam-Au
4
Spin Resist
STD resist, 1 pm
coater
5
Prebake
30 min
6
Expose
7
Develop
oven
Defines hinges and electrodes
Mask 2
photo-wet-I
8 Spin Rinse Dry
Visual Inspec9 tion
10
Postbake
EV1
SRD-Au
Check closest features
30min
oven
continued on next page
137
Table D. I. continued.
11
Cr etch
CR-7
acid-hood2
12
Au etch
Transene gold etch
acid-hood2
13
Strip Resist
nanostrip
14
Dehydrate
1 hour at 150C
acid-hood2
VarTemp
Oven
15
Spin SU-8
15 microns thick
16
Softbake
17
Expose
18
Post Exp Bake
19
Develop
20
21
Rinse
Visual Inspection
22
Hardbake
23
Cleave Wafer
24
XeF2 release
SU8-spinner
hotplate/SU8Oven
Mask 3
Defines SU-8 structural
elements
EV1
Further crosslinks SU-8
hotplate
photo-wet-Au
Use fresh PM Acetate
photo-wet-Au
Check closest features
hotplate
use scribe/tweezer
About 30 minutes
XeF2
138
Appendix E
Mask Layout for SecondGeneration Supercapacitor Devices
DnO
flu
]D
Enfl
Figure E- 1: Five-flap device with current loop design for Lorentz force-actuated folding via
rotating magnetic field.
139
F
0
0
0
0
0_
D-
Figure E-2: One-flap supercapacitor device with etch holes designed to reduce strain on the gold
hinges. The wide, center hinge also acts to reduce hinge damage.
0
030
0
0
0 0
0
0
0
0
0
0
0
0 110
Figure E-3: Same design as Figure E-2 except with even more etch holes for faster release.
140
Figure E-4: One-flap device with current loop design for Lorentz force-actuated folding via
rotating magnetic field.
a
0
13
0
a
13
D17
D
D
Figure E-5: Supercapacitor device with two electrochemical cells stacked on top of each other
141
DOO
0000
000 0
0000O
0000O
Figure E-6: Supercapacitor device with two electrochemical cells right next to each other.
142
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